eglfs: find correct framebuffer index even if device node is symlink

Using the Vivante driver on a board with different device trees I found the need
to let udev point me to the framebuffer actually connected to HDMI by adding a
symlink. Since the extraction of the framebuffer index failed and always
returned 0 the GUI still always showed up on the first framebuffer.

Change-Id: Ib4aa0fdd6e85d296c17fd977921cbc78e52dcdcf
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/plugins/platforms/eglfs/api/qeglfsdeviceintegration.cpp

@@ -52,6 +52,7 @@
 #include <QScreen>
 #include <QDir>
 #if QT_CONFIG(regularexpression)
+#  include <QFileInfo>
 #  include <QRegularExpression>
 #endif
 #include <QLoggingCategory>
@@ -144,7 +145,12 @@ int QEglFSDeviceIntegration::framebufferIndex() const
     int fbIndex = 0;
 #if QT_CONFIG(regularexpression)
     QRegularExpression fbIndexRx(QLatin1String("fb(\\d+)"));
-    QRegularExpressionMatch match = fbIndexRx.match(QString::fromLocal8Bit(fbDeviceName()));
+    QFileInfo fbinfo(QString::fromLocal8Bit(fbDeviceName()));
+    QRegularExpressionMatch match;
+    if (fbinfo.isSymLink())
+        match = fbIndexRx.match(fbinfo.symLinkTarget());
+    else
+        match = fbIndexRx.match(fbinfo.fileName());
     if (match.hasMatch())
         fbIndex = match.captured(1).toInt();
 #endif