drm/virtio: only destroy created contexts

This can happen if userspace doesn't issue any 3D ioctls before
closing the DRM fd.

Fixes: 72b48ae8 ("drm/virtio: enqueue virtio_gpu_create_context after the first 3D ioctl")
Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200408232938.55816-1-gurchetansingh@chromium.org


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