compositor-drm: drm_output_apply_state_legacy heads

Fix this function to support more than one head per output.

v9:
- Change { connectors, 0 } to { NULL, 0 } in drmModeSetCrtc() args.

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