drm/amd/powerplay: fix bug that get wrong polaris evv voltage.

value is 32 bits for polaris, not 16.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>