drm/nouveau: don't use the default pll limits in table v2.1 on nv50+ cards

This fixes issues bug 30370 and prevents another possible divide by zero on
the original nv50 cards, by returning -ENOENT

Signed-off-by: Emil Velikov <eeydev@nottingham.ac.uk>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>