drm/amd/powerplay: fix typos of volt/millivolt symbols in comment.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>