drm/nvd0/disp: calculate U script id in supervisor interrupt

This is like we do on nv50:nvd9 already.  There's been no problems seen
yet with using this *seemingly* scratch register to store the value, but
we won't be able to do this anymore once nv50's code is merged.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>