drm/nouveau/gr/gf100-: calculate and use sm mapping table

drm/nouveau/gr/gf100-: calculate and use sm mapping table

There's a number of places that require this data, so let's separate out
the calculations to ensure they remain consistent.

This is incorrect for GM200 and newer, but will produce the same results
as we did before.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>