xcb: clear m_singlescreen before update

Change-Id: Iafff57be82b5beb1f5702c00e0b6d7d0dd3ccdc2
Reviewed-by: Liang Qi <liang.qi@qt.io>
(cherry picked from commit fd9aeb1b38e1b42d497723457317ba1b0cfeef8e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/xcb/qxcbscreen.cpp

@@ -598,6 +598,7 @@ void QXcbScreen::setMonitor(xcb_randr_monitor_info_t *monitorInfo, xcb_timestamp
     m_crtcs.clear();
     m_output = XCB_NONE;
     m_crtc = XCB_NONE;
+    m_singlescreen = false;
 
     if (!monitorInfo) {
         m_monitor = nullptr;