drm/amd/display: fix trigger_hotplug to support mst case

[Why & How]
Correct few problems below to have debugfs trigger_hotplug entry
supports mst case

* Adjust the place for acquiring the hpd_lock. We'll also access
  dc_link when simulate unplug
* When detect the connector is a mst root, call
  reset_cur_dp_mst_topology() to simulate unplug
* Don't support hotplug caused by CSN message since we can't change
  mst topology info directly. We can't simulate that
* Clean up redundant code

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>