drm/radeon/kms/pm: Disable voltage adjust on RS780/RS880

The vddc value in the power tables is not an actual voltage
like on discrete r6xx/r7xx/evergreen systems, but instead has
a symbolic meaning (e.g., NONE, LOW, HIGH, etc.).  See atombios.h

Most RS780/RS880 vbioses don't have a SetVoltage table anyway,
so it shouldn't be doing anything to the hardware at the moment.

I need to figure out how voltage is supposed to work on the newer
IGPs; until then, disable it.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>