drm/doc: polish for sturct drm_connector

- switch everything over to inline comments
- add notes about locking, links to functions and other related stuff
- also include a note about Ville's soon-to-be-merged
  drm_connector_for_each_possible_encoder().

Also check that all the hyperlinks in drm_connector.h work and fix
them as needed.
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-5-daniel.vetter@ffwll.ch