drm/cirrus: bind also to qemu-xen-traditional

qemu as used by xend/xm toolstack uses a different subvendor id.
Bind the drm driver also to this emulated card.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>