drm/nouveau/pm: add initial NV3x/NVCx memtiming support, improve other cards

NV30: Create framework for memtm
NV50: Improve reg creation,
NV50: Use P.version instead of card codename/stepping,
NVC0: Initial memtiming code for Fermi,
Renamed regs for consistency,
Overall redesign to improve readability,
Avoid kfree on null-pointer

Signed-off-by: Roy Spliet <r.spliet@student.tudelft.nl>