Fix font DPI loading and saving for wayland
On wayland a separate config value was used from X11. The rationale being if you're using wayland screen scaling, we don't want to scale fonts as well, it would cause a double scaling. (plasma-desktop fbbdfa100128e9c4cc12b37854a4a054562983bc)
In the port to kcfg this got broken. The loading is also somewhat broken.