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.

Note: drm_backend_update_unused_outputs() seems to conflict with
drm_output_enable()/drm_output_disable() on when a connector ID belongs
in the unused_connectors array. This patch is preserving the old
semantics and confusion.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>