compositor-drm: start migration to head-based output API

Hook up the libweston facing head-based output API by introducing struct
drm_head, but leave it as a fake so that members can be migrated in
pieces in follow-up patches.

The DRM backend continues to create an output for each connected
connector only, and during output creation it also creates a drm_head
for it. This allows it to pretend it supports the head-based output API
as long as there is only one head per output ever attached.

create_output callback is fake, it will only look up the existing
drm_output by the head name.

Clones are not yet supported, hence max is defined to 1.

This unfortunately introduces some temporary code that will be revomed
later, but seems to be necessary to avoid a single big patch.

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