compositor-drm: use head_find_by_connector in update_unused_outputs

Making this function not depend on drm_head::output field through
drm_output_find_by_connector() will later allow to remove the
drm_head::output field before removing the unused_connectors array. This
helps keeping the commit more fine-grained.

drm_backend_update_unused_outputs() was only interested in enabled
outputs. The new code is 100% equivalent to the old code. The
difference is that weston_head::output is only set for attached heads. A
connector cannot be in use if it is not attached to an output.

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>