drm/doc: Fix more kerneldoc/sphinx warnings

These are the leftovers I could only track down using keep_warnings =
True. For some of them we might want to update our style guide on how
to reference structures and constants, not sure ...

Cc: Markus Heiser <markus.heiser@darmarit.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-1-git-send-email-daniel.vetter@ffwll.ch