drm/nva3/pm: another few magic regs, and slightly better 0x004018 handling

Not entirely convinced 0x004018 transitions are correct yet, but, it's
an improvement.

The 750MHz value comes from fiddling with the binary driver + coolbits on
two different DDR3 NVA8 chipsets (T510 NVS3100M, and NVS300), not a clue
where this number comes from.

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