drm/virtio: use fence_id when processing fences

Currently, the fence ID, which can be used to identify a
virtgpu fence, is the same as the fence sequence number.

Let's use the fence_id name to clearly signal this.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201119010809.528-4-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>