drm/amd/powerplay: Remove unused variable causing compile warning

If treating warnings as errors this causes a build error

Signed-off-by: Jordan Lazare <Jordan.Lazare@amd.com>
Reviewed-by: Alex Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>