Fix display recognition for IBUS

The parsing of the DISPLAY environment variable was wrong,
and only worked by accident for DISPLAY=':[0-9]'.

Task-number: QTBUG-62068
Change-Id: I6860e3907c9b1ad6e538d1b5d08628cd306b4aa1
Reviewed-by: Alexander Volkov <a.volkov@rusbitech.ru>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/plugins/platforminputcontexts/ibus/qibusplatforminputcontext.cpp

@@ -647,7 +647,7 @@ QString QIBusPlatformInputContextPrivate::getSocketPath()
     if (pos2 > 0)
         displayNumber = display.mid(pos, pos2 - pos);
     else
-        displayNumber = display.right(pos);
+        displayNumber = display.mid(pos);
     if (debug)
         qDebug() << "host=" << host << "displayNumber" << displayNumber;