drm: kerneldoc for drm_fops.c

Just prep work before I throw more drm_event refactorings on top.

Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452548477-15905-2-git-send-email-daniel.vetter@ffwll.ch


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>