Disable Arbitrary Mipmap Detection checkbox when GPU Texture Decoding is enabled

See https://bugs.dolphin-emu.org/issues/13049

Source/Core/DolphinQt/Config/Graphics/GraphicsWindow.h

@@ -25,6 +25,7 @@ public:
 signals:
   void BackendChanged(const QString& backend);
   void UseFastTextureSamplingChanged();
+  void UseGPUTextureDecodingChanged();
 
 private:
   void CreateMainLayout();