drm/radeon: fix voltage setup on hawaii

Missing parameter when fetching the real voltage values
from atom.  Fixes problems with dynamic clocking on
certain boards.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=87457



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org