drm/amd/powerplay: fix segment fault issue in multi-display case.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org