drm/DocBook: Add more drm_bridge documentation

Add DOC sections giving an overview of drm_bridge and how to fill up the
drm_bridge_funcs ops. Add these to drm.tpml in DocBook.

Add headerdocs for funcs in drm_bridge.c that don't have them yet.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
[danvet: Amend kerneldoc as discussed with Archit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>