drm: Update kerneldoc for drm_crtc.[hc]

After going through all the trouble of splitting out parts from
drm_crtc.[hc] and then properly documenting each I've entirely
forgotten to show the same TLC for CRTCs themselves!

Let's make amends asap.

v2: Review from Eric.

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>