drm/amd/display: clear remote dc_sink when stop mst

[Why]
Currently, we don't have code path to release remote dc_sink when unplug
MST hub from the system. After few times hotplug, we hit the limition of
maximum number of remote dc_sink and can't light up new connected monitor
anymore.

[How]
Releasing all remote dc_sink at dm_helpers_dp_mst_stop_top_mgr() was
removed by previous patch. Restore it.

Reviewed-by: Jerry Zuo <Jerry.Zuo@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>