drm/nouveau/volt: Make use of cvb coefficients

I'm quite sure that those coefficients are real close, because while
testing the biggest error compared to nvidia was around -1.5% (biggest
error with right coefficients is 12.5mV / 600mV = 2%).

These coefficients were REed by modifing the voltage map entries and by
calculating the set voltage back until I was able to forecast which voltage
nvidia sets for a given voltage map entry.

With these formulars I am able to precisely predict at which exact
temperature Nvidia down- or upvolts due to a changed therm reading.

That's why I am quite sure these are right, or at least really really
close.

v4: Use better coefficients and speedo.
v5: Add error message when speedo is missing.

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>