drm/connector: document DRM_MODE_COLORIMETRY_COUNT

The drm_colorspace enum member DRM_MODE_COLORIMETRY_COUNT has been
properly documented by moving the description out of the enum to the
member description list to get rid of an additional warning and improve
documentation clarity.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230906-topic-drm_connector_doc-v2-1-1f2dcaa43269@gmail.com