drm/virtio: drop virtio_gpu_output->enabled

Not needed, already tracked by drm_crtc_state->active.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20200818072511.6745-3-kraxel@redhat.com