drm/amd/powerplay: get real display device num by cgs interface

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>

Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>