drm/nouveau/gr: add GM20B support

Add support for GM20B's graphics engine, based on GK20A. Note that this
code alone will not allow the engine to initialize on released devices
which require PMU-assisted secure boot.

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