drm/radeon/kms: handle special cases for vddc

A voltage value of 0xff01 requires that the driver
look up the max voltage for the board based using the
atom SetVoltage command table.

Setting the proper voltage should fix stability on
some newer asics.

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