When the backend sends VHOST_USER_GPU_DMABUF_SCANOUT2, handle it
by getting the modifiers information which is now available.

Signed-off-by: Erico Nunes <ernunes@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20230714153900.475857-4-ernunes@redhat.com>