compositor-drm: allow shared-CRTC cloning

Allow cloning up to 4 connectors from the same CRTC. All the
implementation bits support more than one head per output already.

Four is just an arbitary number, small but unlikely to ever be the
limiting factor in cloning since hardware is usually very restricted.

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