drm/nouveau/clk: Don't create cstates with voltages higher than what the gpu can do

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