drm: add usb framework

This adds an initial framework to plug USB graphics devices
into the drm/kms subsystem.

I've started writing a displaylink driver using this interface.

Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_stub.c

@@ -463,6 +463,7 @@ void drm_put_dev(struct drm_device *dev)
 
 	drm_put_minor(&dev->primary);
 
+	list_del(&dev->driver_item);
 	if (dev->devname) {
 		kfree(dev->devname);
 		dev->devname = NULL;