drm/nouveau/fifo: add RAMFC info to nvkm_chan_func

- adds support for specifying SUBDEVICE_ID for channel
- rounds non-power-of-two GPFIFO sizes down, rather than up

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