drm/nouveau/volt/gk104: add support for pwm and gpio modes

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: Martin Peres <martin.peres@free.fr>