compositor-drm: remove unused_connectors array

Replace the unused_connectors array by iterating through the head list
instead. A head that is not enabled (attached to an enabled output) is
basically an unused connector.

All connectors regardless of their status have a drm_head. This has the
nice effect that drm_pending_state_apply_atomic() does not need to
re-query the connector properties every time, they can be simply looked
up in the 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>