Qt/Settings: Make GPU adapter more sensible

Add a default entry, rather than leaving it blank.

src/duckstation-qt/gpusettingswidget.h

@@ -18,6 +18,7 @@ public:
 private Q_SLOTS:
   void updateScaledDitheringEnabled();
   void populateGPUAdapters();
+  void onGPUAdapterIndexChanged();
 
 private:
   void setupAdditionalUi();