drm/nouveau: add support for GB20x

This commit adds support for the GB20x GPUs found on GeForce RTX 50xx
series boards.

Beyond a few miscellaneous register moves and HW class ID plumbing,
this reuses most of the code added to support GH100/GB10x.

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>