feat(nvidia): add enable option

modules/nvidia/default.nix

@@ -1,6 +1,9 @@
-{ config, ... }:
+{ lib, config, ... }:
 
 {
+  options.nvidia.enable = lib.mkEnableOption "NVIDIA graphics";
+
+  config = lib.mkIf config.nvidia.enable {
     nixpkgs.allowedUnfree = [
       "nvidia-x11"
       "nvidia-persistenced"
@@ -19,4 +22,5 @@
     };
 
     services.xserver.videoDrivers = [ "nvidia" ];
+  };
 }