compositor-drm: Ignore views on other outputs

When we come to assign_planes, try very hard to ignore views which are
only visible on other outputs, rather than forcibly moving them to the
primary plane, which causes damage all round and unnecessary repaints.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Emre Ucan <eucan@de.adit-jv.com>