drm: vkms: select DRM_KMS_HELPER

Without this, we get link errors during randconfig build:

drivers/gpu/drm/vkms/vkms_drv.o:(.rodata+0xa0): undefined reference to `drm_atomic_helper_check'
drivers/gpu/drm/vkms/vkms_drv.o:(.rodata+0xa8): undefined reference to `drm_atomic_helper_commit'
drivers/gpu/drm/vkms/vkms_plane.o:(.rodata+0x0): undefined reference to `drm_atomic_helper_update_plane'
drivers/gpu/drm/vkms/vkms_plane.o:(.rodata+0x8): undefined reference to `drm_atomic_helper_disable_plane'
drivers/gpu/drm/vkms/vkms_plane.o:(.rodata+0x18): undefined reference to `drm_atomic_helper_plane_reset'
drivers/gpu/drm/vkms/vkms_plane.o:(.rodata+0x28): undefined reference to `drm_atomic_helper_plane_duplicate_state'
drivers/gpu/drm/vkms/vkms_plane.o:(.rodata+0x30): undefined reference to `drm_atomic_helper_plane_destroy_state'
drivers/gpu/drm/vkms/vkms_output.o:(.rodata+0x1c0): undefined reference to `drm_helper_probe_single_connector_modes'
drivers/gpu/drm/vkms/vkms_crtc.o:(.rodata+0x40): undefined reference to `drm_atomic_helper_crtc_reset'
drivers/gpu/drm/vkms/vkms_crtc.o:(.rodata+0x70): undefined reference to `drm_atomic_helper_set_config'
drivers/gpu/drm/vkms/vkms_crtc.o:(.rodata+0x78): undefined reference to `drm_atomic_helper_page_flip'
drivers/gpu/drm/vkms/vkms_crtc.o:(.rodata+0x90): undefined reference to `drm_atomic_helper_crtc_duplicate_state'
drivers/gpu/drm/vkms/vkms_crtc.o:(.rodata+0x98): undefined reference to `drm_atomic_helper_crtc_destroy_state'

Fixes: 854502fa ("drm/vkms: Add basic CRTC initialization")
Fixes: 1c7c5fd9 ("drm/vkms: Introduce basic VKMS driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709154901.1989316-1-arnd@arndb.de