drm/virtio: get rid of drm_encoder_to_virtio_gpu_output

Not used anywhere.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20191219005733.18960-3-gurchetansingh@chromium.org


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