virtio-gpu: use no-merge for fill-modes

Avoid the sticky preferred mode bit by using the no-merge version of the
function (this allows gnome-shell to resize to lower resolutions than
the default resolution)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>