drm/radeon/kms/r600+: use voltage from requested clock mode (v3)

This fixes FDO bug #28375, it's kind of regression, so quite important to have
it for .35.

V2: Fix on RV770+ as well. All other chipsets have only one clock mode per
state.

V3: I'm out of luck today. Grepped for voltage in r*.c and missed evergreen.

agd5f: rebased

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>