drm/nouveau/kms: add support for GB20x

Adds basic support for the new display classes available on GB20x GPUs.

Most of the changes here deal with HW method moves, with the only other
change of note being tweaks to skip allocation of CTXDMA objects, which
aren't required on Blackwell display.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>