input: Make sure input backends are initialised when the workspace is set up

The outputs already present upon workspace setup wouldn't signal. This
was easily triggered running a standalone kwin session on a tty, it
would manifest with the touchscreen not working complaining that it
didn't have an output assigned yet.

BUG: 466721

src/input.cpp

@@ -2636,6 +2636,7 @@ void InputRedirection::setupWorkspace()
 
         setupTouchpadShortcuts();
         setupInputFilters();
+        updateScreens();
     }
 }