drm/nouveau/fifo: add GK20A support

GK20A's FIFO is compatible with NVE0, but only features 128 channels and
1 runlist.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>