drm/doc: Update kerneldoc for drm_crtc.h

Apparently not everyone has been super dutiful with updating this
stuff.

I still decided to leave out the documentation for all the *_property
pointers we have in drm_mode_config.

v2: Feedback from Liviu.

Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>