drm/nouveau/gm204/6: add voltage control using the new gk104 volt class

I got confirmation that we can read and change the voltage with the same code.
The divider is also computed correctly on the gm204 we got our hands on.

Thanks to Yoshimo on IRC for executing the tests on his gm204!

Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>