weston: migrate RDP to head-based output API

Migrate the RDP frontend to use the new head-based output configuration
API: listen for heads_changed, and process all heads.

v7:
- remove unnecessary 'goto out' in load_rdp_backend()

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