drm/amd/powerplay: correct smc display config for multi monitor

Need to take into account multi-head with synced displays.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>