drm/panfrost: fix -Wmissing-prototypes warnings

We get these warnings when build kernel W=1:
drivers/gpu/drm/panfrost/panfrost_perfcnt.c:35:6: warning: no previous prototype for ‘panfrost_perfcnt_clean_cache_done’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_perfcnt.c:40:6: warning: no previous prototype for ‘panfrost_perfcnt_sample_done’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_perfcnt.c:190:5: warning: no previous prototype for ‘panfrost_ioctl_perfcnt_enable’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_perfcnt.c:218:5: warning: no previous prototype for ‘panfrost_ioctl_perfcnt_dump’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_perfcnt.c:250:6: warning: no previous prototype for ‘panfrost_perfcnt_close’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_perfcnt.c:264:5: warning: no previous prototype for ‘panfrost_perfcnt_init’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_perfcnt.c:320:6: warning: no previous prototype for ‘panfrost_perfcnt_fini’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_mmu.c:227:6: warning: no previous prototype for ‘panfrost_mmu_flush_range’ [-Wmissing-prototypes]
drivers/gpu/drm/panfrost/panfrost_mmu.c:435:5: warning: no previous prototype for ‘panfrost_mmu_map_fault_addr’ [-Wmissing-prototypes]

For file panfrost_mmu.c, make functions static to fix this.
For file panfrost_perfcnt.c, include header file can fix this.

Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Reviewed-by: Steven Price <steven.price@arm.com>
Cc: stable@vger.kernel.org
[robh: fixup function parameter alignment]
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1571967015-42854-1-git-send-email-wang.yi59@zte.com.cn