compositor-drm: Add modifiers to GBM dmabuf import

Add support for the GBM_BO_IMPORT_FD_MODIFIER path, which allows us to
import multi-plane dmabufs, as well as format modifiers.

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>
Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/113