drm/nouveau/fb/ramnv50: Voltage GPIOs

Does not seem to be necessary for NVA0, hence untested by me.

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Tested-by: Pierre Moreau <pierre.morrow@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>