drm/nouveau/pm: translate ramcfg strap through ram restrict table

Hopefully this is how we're supposed to correctly handle when the RAMCFG
strap is above the number of entries in timing-related tables.

It's rather difficult to confirm without finding a configuration where
the ram restrict table doesn't map 8-15 back onto 0-7 anyway.  There's
not a single vbios in the repo which is configured differently..

In any case, this is probably still better than potentially reading
outside of the bounds of various tables..

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