drm/amd/powerplay: fixed uninitialized value

The 'result' is not initialized correctly. It causes the API
return an error code even on success.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org