drm/nouveau: make the behaviour of get_pll_limits() consistent

This replaces all the pll_types definitions for ones that match the types
used in the tables in recent VBIOS versions.

get_pll_limits() will now accept either type or register value as input
across all limits table versions, and will store the actual register ID
that a PLL type refers to in the returned structure.

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