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.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>