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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_debugfs.c

@@ -1256,14 +1256,22 @@ static ssize_t trigger_hotplug(struct file *f, const char __user *buf,
 		return -EINVAL;
 	}
 
+	kfree(wr_buf);
+
 	if (param_nums <= 0) {
 		DRM_DEBUG_DRIVER("user data not be read\n");
-		kfree(wr_buf);
+		return -EINVAL;
+	}
+
+	mutex_lock(&aconnector->hpd_lock);
+
+	/* Don't support for mst end device*/
+	if (aconnector->mst_port) {
+		mutex_unlock(&aconnector->hpd_lock);
 		return -EINVAL;
 	}
 
 	if (param[0] == 1) {
-		mutex_lock(&aconnector->hpd_lock);
 
 		if (!dc_link_detect_sink(aconnector->dc_link, &new_connection_type) &&
 			new_connection_type != dc_connection_none)
@@ -1300,6 +1308,10 @@ static ssize_t trigger_hotplug(struct file *f, const char __user *buf,
 
 		amdgpu_dm_update_connector_after_detect(aconnector);
 
+		/* If the aconnector is the root node in mst topology */
+		if (aconnector->mst_mgr.mst_state == true)
+			reset_cur_dp_mst_topology(link);
+
 		drm_modeset_lock_all(dev);
 		dm_restore_drm_connector_state(dev, connector);
 		drm_modeset_unlock_all(dev);
@@ -1310,7 +1322,6 @@ static ssize_t trigger_hotplug(struct file *f, const char __user *buf,
 unlock:
 	mutex_unlock(&aconnector->hpd_lock);
 
-	kfree(wr_buf);
 	return size;
 }