drm/nouveau/gr/gv100-: port smid mapping code from nvgpu

Essentially ripped verbatim from NVGPU, comments and all, and adapted to
nvkm's structs and style.

- maybe fixes an nvgpu bug though, a small tweak was needed to match RM

v2:
- remove unnecessary WARN_ON

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>