drm/nouveau/graph: add GK20A support

Add a GR device for GK20A based on NVE4, with the correct classes
definitions (GK20A's 3D class is 0xa297).

Most of the NVE4 code can be used on GK20A, so make relevant bits of
NVE4 available to other chips as well.

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