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.

BUG: 441043