drm/udl: only bind to the video devices on the hub.

commit e5a867a51d9b009f90d5dca6a320608e4e8a37ec upstream.

This is ported from udlfb.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=832188
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>