compositor-drm: drm_output_find_by_connector from head_list

Switch drm_output_find_by_connector() to search for the output by
iterating the compositor's head_list. drm_head_find_by_connector() will
be useful later on its own.

As of "compositor-drm: start migration to head-based output API" the
head list is guaranteed to contain all created drm_outputs through the
automatically created drm_head.

This simplifies the code a little, introduces
drm_head_find_by_connector(), and works towards the eventual removal of
drm_output_find_by_connector().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>