drm/nouveau/fifo: add USERD info to nvkm_chan_func

And use it to cleanup multiple implementations of almost the same thing.

- prepares for non-polled / client-provided USERD
- only zeroes relevant "registers", rather than entire USERD

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>