drm/amd/display: Apply fake sink back to MST sequence

drm/amd/display: Apply fake sink back to MST sequence

[Why]
It fixes the failure to create stream for sink in the scenario
when hotplug SST and MST in sequence, and disconnect MST.

[How]
Add the fake sink back after the majority of MST rework is done.

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>