drm/nouveau/pm: calculate memory timings at perflvl creation time

Statically generating the PFB register and MR values for each timing set
turns out to be insufficient.  There's at least one (so far) known piece
of information which effects MR values which is stored in the perflvl
entry on some chipsets (and in another table on later ones), which is
disconnected from the timing table entries.

After this change we will generate a timing set based on an input clock
frequency instead, and have this data stored in the performance level
data.

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