nvkm_volt_map_min is a copy of nvkm_volt_map, which always returns the
lowest possible voltage for a cstate.

nvkm_volt_map will get a temperature parameter there later and also fix
the voltage calculation, so that this functions will be completly
different later.
Signed-off-by: NKarol Herbst <karolherbst@gmail.com>
Reviewed-by: NMartin Peres <martin.peres@free.fr>
Tested-by: NPierre Moreau <pierre.morrow@free.fr>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>