drm/nouveau/gr/fuc: Store $r0 in interrupt handler

It's supposed to always be 0, but at least nv_iowr() temporarily violates
this. Since the ih touches $r0, it should be stored.

Signed-off-by: Roy Spliet <rs855@cam.ac.uk>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>