drm/nouveau/volt: save the voltage range we are able to set

We shouldn't set voltages below the min or above the max voltage the gpu is
able to set, so save the range for future lookups.

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