nixos/plasma6: default to Wayland for SDDM

While technically still experimental, it seems to produce less
issues than X11 at this point, and there's a weird issue with
X11 setups that we've been unable to track down that really
hurts the new user experience, so let's just go with this.

nixos/modules/services/desktop-managers/plasma6.nix

@@ -302,7 +302,10 @@ in
     services.displayManager.sddm = {
       package = kdePackages.sddm;
       theme = mkDefault "breeze";
-      wayland.compositor = "kwin";
+      wayland = mkDefault {
+        enable = true;
+        compositor = "kwin";
+      };
       extraPackages = with kdePackages; [
         breeze-icons
         kirigami