drm: Improve kerneldoc for new mode object refcounting

Slipped through the cracks in my review. The one issue I spotted
is that drm_mode_object_find now acquires references and can be
used on FB objects, which caused follow-on bugs in get/set_prop ioctls.
Follow-up patches will fix that.

[airlied: fixup some incr fb/decr object mixups]

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>