backwards: Adjust to updated nixpkgs

hosts/backwards/modules/gnome/default.nix

@@ -8,6 +8,14 @@
       };
     };
 
+    desktopManager = {
+      gnome.enable = { };
+    };
+
+    desktopManager = {
+      gdm.enable = { };
+    };
+
     xserver = {
       enable = true;
 
@@ -15,14 +23,6 @@
         layout = "se";
         variant = "";
       };
-
-      desktopManager = {
-        gnome.enable = true;
-      };
-
-      displayManager = {
-        gdm.enable = true;
-      };
     };
   };