docs: gpu: i915.rst: Fix several C duplication warnings

As reported by Sphinx:

	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:1147: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_oa_wait_unlocked'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:1169: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_oa_poll_wait'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:1189: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_oa_read'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:2669: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_oa_stream_enable'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:2734: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_oa_stream_disable'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:2820: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_oa_stream_init'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3010: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_read'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3098: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_poll_locked'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3129: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_poll'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3152: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_enable_locked'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3181: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_disable_locked'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3273: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_ioctl'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3296: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_destroy_locked'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3321: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_release'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3379: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_open_ioctl_locked'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3534: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'read_properties_unlocked'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3717: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_open_ioctl'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3760: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_register'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:3789: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_unregister'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:4009: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_add_config_ioctl'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:4162: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_remove_config_ioctl'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:4260: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_init'.
	./Documentation/gpu/i915:646: ./drivers/gpu/drm/i915/i915_perf.c:4423: WARNING: Duplicate C declaration, also defined in 'gpu/i915'.
	Declaration is 'i915_perf_fini'.

With Sphinx 3, C declarations can't be duplicated anymore,
so let's exclude those from the other internals found on
i915_perf.c file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>