drm: add drm_dev_alloc() helper

Instead of managing device allocation+initialization in each bus-driver,
we should do that in a central place. drm_fill_in_dev() already does most
of it, but also requires the global drm lock for partial AGP device
registration.

Split both apart so we have a clean device initialization/allocation
phase, and a registration phase.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_usb.c b/drivers/gpu/drm/drm_usb.c
index 8766472..34ad8ed 100644
--- a/drivers/gpu/drm/drm_usb.c
+++ b/drivers/gpu/drm/drm_usb.c
@@ -7,18 +7,15 @@
 		    struct drm_driver *driver)
 {
 	struct drm_device *dev;
-	struct usb_device *usbdev;
 	int ret;
 
 	DRM_DEBUG("\n");
 
-	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
+	dev = drm_dev_alloc(driver, &interface->dev);
 	if (!dev)
 		return -ENOMEM;
 
-	usbdev = interface_to_usbdev(interface);
-	dev->usbdev = usbdev;
-	dev->dev = &interface->dev;
+	dev->usbdev = interface_to_usbdev(interface);
 
 	mutex_lock(&drm_global_mutex);