drm/doc: integrate crtc helper api into docbook

- Add the missing doc for drm_helper_move_panel_connectors_to_head.
- Fixup any outdated stuff in existing sections. I've only looked at
  those kerneldoc headers that actually resulted in a complaint from
  the kerneldoc parser tool.

v2:
- Actually include the docbook snippet in the right patch.
- Fix spelling fail.

v3: It's now called drm_crtc_helper_set_mode, spotted by Laurent
Pinchart.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>