drm/nouveau/ibus: add GK20A support

Add support for initializing the priv ring of GK20A. This is done by the
BIOS on desktop GPUs, but needs to be done by hand on Tegra.

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