compositor-drm: move connector fields into drm_head

Move the connector related fields from drm_output to the drm_head. A
drm_head represents a connector for now.

The code in drm_head_create() to update connector data, monitor
information, etc. is moved into a new function. This will be useful when
DRM-backend starts creating heads for all connectors regardless of their
connection status and will need to update them on hotplug events.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>