modules: sddm is no longer the config wide default

Make room for a GDM option as well (SDDM does not start on my
NVIDIA+Wayland config)

modules/gdm.nix

@@ -0,0 +1,23 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}: let
+  inherit
+    (lib)
+    mkEnableOption
+    mkIf
+    ;
+
+  cfg = config.my.displayManager.gdm;
+in {
+  options.my.displayManager.gdm.enable = mkEnableOption "GDM setup";
+
+  config = mkIf cfg.enable {
+    services.displayManager.gdm = {
+      enable = true;
+      wayland = true;
+    };
+  };
+}