drm: Docbook fixes

Bunch of small leftovers spotted by looking at the make htmldocs output.

I've left out dp mst, there's too much amiss there.

v2: Also add the missing parameter docbook in the dp mst code - Dave
Airlie correctly pointed out that we don't actually want kerneldoc for
the missing structure members in header files.

Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>