Config: Port Input setting to new config system.

Source/Core/Core/HW/VideoInterface.cpp

@@ -881,7 +881,7 @@ void Update(u64 ticks)
 
   if (s_half_line_of_next_si_poll == s_half_line_count)
   {
-    Core::UpdateInputGate(!SConfig::GetInstance().m_BackgroundInput,
+    Core::UpdateInputGate(!Config::Get(Config::MAIN_INPUT_BACKGROUND_INPUT),
                           SConfig::GetInstance().bLockCursor);
     SerialInterface::UpdateDevices();
     s_half_line_of_next_si_poll += 2 * SerialInterface::GetPollXLines();