Some scanouts during boot are top-down without it.

y0_top is set from VHOST_USER_GPU_DMABUF_SCANOUT code path in the last
patch of this series.

In current QEMU code base, only vfio/display uses dmabuf API. But the
VFIO query interface doesn't provide or need that detail so far.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180713130916.4153-5-marcandre.lureau@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>