drm/virtio: Fix double free on probe failure

The virtio_gpu_init() will free vgdev and vgdev->vbufs on failure.
But such failure will be caught by virtio_gpu_probe() and then
virtio_gpu_release() will be called to do some cleanup which
will free vgdev and vgdev->vbufs again. So let's set dev->dev_private
to NULL to avoid double free.
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210517084913.403-2-xieyongji@bytedance.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_kms.c

@@ -234,6 +234,7 @@ int virtio_gpu_init(struct drm_device *dev)
 err_vbufs:
 	vgdev->vdev->config->del_vqs(vgdev->vdev);
 err_vqs:
+	dev->dev_private = NULL;
 	kfree(vgdev);
 	return ret;
 }