drm/amd/display: Clear dc remote sinks on MST disconnect

[Why&How]
Recent changes to upstream mst code remove the callback which
cleared the internal state for mst. Move the missing functionality
that was previously called through the destroy call back for mst connector
destroy

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>