drm: Add kernel-doc for drm_crtc_commit_get/put

I was lazy, rectify that! Also align with drm_atomic_state_get/put for
ocd.

v2: Git add helps.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161221130335.5321-1-daniel.vetter@ffwll.ch