drm/virtgpu: fix double unregistration

drm_put_dev also calls drm_dev_unregister, so dev will be unregistered
twice.
Replace it with drm_dev_put to fix it.
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191109075417.29808-1-hslester96@gmail.com

drivers/gpu/drm/virtio/virtgpu_drv.c

@@ -137,7 +137,7 @@ static void virtio_gpu_remove(struct virtio_device *vdev)
 
 	drm_dev_unregister(dev);
 	virtio_gpu_deinit(dev);
-	drm_put_dev(dev);
+	drm_dev_put(dev);
 }
 
 static void virtio_gpu_config_changed(struct virtio_device *vdev)