drm/amd/display: Use |= when assigning wm_optimized_required

[Description]
Use |= when assigning to wm_optimized required because
we don't want to overwrite the flag in case the optimize
did not happen yet

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>