compositor-drm: find disconnects from head_list

Instead of iterating output_list and pending_output_list, iterate
head_list to find outputs whose connectors have been disconnected.

This helps a following patch to move connector fields from drm_output to
drm_head.

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>