drm/cma-helpers: Use recommened kerneldoc for struct member refs

drm/cma-helpers: Use recommened kerneldoc for struct member refs

I just learned that &struct_name.member_name works and looks pretty
even. It doesn't (yet) link to the member directly though, which would
be really good for big structures or vfunc tables (where the
per-member kerneldoc tends to be long).

Also some minor drive-by polish where it makes sense, I read a lot
of docs ...

v2: Review from Laurent:
- Move misplaced doc change to the right patch.
- Remove "DRM driver's", it's redundant.
- Spotted 3 more places where where we could add prose reference with
  a real one.

Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483044517-5770-14-git-send-email-daniel.vetter@ffwll.ch