drm/nouveau: Associate memtimings with performance levels on cards <= nv98

v2 (Ben Skeggs): fix ramcfg strap, and remove bogus handling of perf 0x40

Signed-off-by: Martin Peres <martin.peres@ensi-bourges.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>