fix: 修复切换显示模式后无法拖拽调整任务栏高度的问题

显示模式切换后未能更新拖拽区域，及时更新一下就好

Log: 修复切换显示模式后无法拖拽调整任务栏高度的问题
Bug: https://pms.uniontech.com/zentao/bug-view-46393.html
Change-Id: I5bf5ebdf9b510bf5dd4d05e5488ab6e378ba2812
Reviewed-on: http://gerrit.uniontech.com/c/dde-dock/+/3680
Reviewed-by: pengwenhao <pengwenhao@uniontech.com>
Reviewed-by: niecheng <niecheng@uniontech.com>
Reviewed-by: fanpengcheng <fanpengcheng@uniontech.com>
Tested-by: fanpengcheng <fanpengcheng@uniontech.com>

frame/util/multiscreenworker.cpp

@@ -452,6 +452,7 @@ void MultiScreenWorker::onDisplayModeChanged()
     emit requestUpdateRegionMonitor();
     emit requestUpdateFrontendGeometry();
     emit requestNotifyWindowManager();
+    emit requestUpdateDragArea();
 }
 
 void MultiScreenWorker::onHideModeChanged()