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)

base/gui-programs.nix

@@ -18,8 +18,6 @@ in {
   };
 
   config = mkIf config.my.gui.enable {
-    my.displayManager.sddm.enable = true;
-
     programs.gnupg.agent = {
       enable = true;
       enableSSHSupport = true;

hosts/boreal/default.nix

@@ -111,5 +111,6 @@
   programs.waybar.enable = true;
   programs.foot.enable = true;
   services.displayManager.sddm.wayland.enable = true;
+  my.displayManager.gdm.enable = true;
   services.power-profiles-daemon.enable = true;
 }

hosts/talos/default.nix

@@ -154,6 +154,7 @@
   };
 
   # Enable the KDE Plasma Desktop Environment.
+  my.displayManager.sddm.enable = true;
   services.desktopManager.plasma6.enable = true;
   services.desktopManager.cosmic.enable = true;
   services.power-profiles-daemon.enable = true;

modules/default.nix

@@ -1,5 +1,6 @@
 {...}: {
   imports = [
+    ./gdm.nix
     ./sddm.nix
     ./secrets
     ./wakeonwlan.nix

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;
+    };
+  };
+}