compositor-drm: Support modifiers for drm_fb

Use the new drmModeAddFB2WithModifiers interface to import buffers with
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>