compositor-drm: Use plane FB-import helper for scanout

Use the same codepath, which has the added advantage of being able to
import dmabufs.

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>