drm/amd/display: Fix MST recognizes connected displays as one

[What]
MST now recognizes both connected displays

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>