We always want a equal or higher voltage than the requested ones, otherwise
nouveau undervolts.
Signed-off-by: NKarol Herbst <nouveau@karolherbst.de>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

Reported-by: NIlia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: NMartin Peres <martin.peres@free.fr>

Most Keplers actually use the GPIO-based voltage management instead of the new
PWM-based one. Use the GPIO mode as a fallback as it already gracefully handles
the case where no GPIOs exist.

All the Maxwells seem to use the PWM method though.

v2:
 - Do not forget to commit the PWM configuration change!
Signed-off-by: NMartin Peres <martin.peres@free.fr>