feat(nvidia-prime): automatic battery-saver specialisation

This enables a boot option that switches off NVIDIA GPU. Allows for battery saving.

@moduon MT-9339

common/gpu/nvidia/prime.nix

@@ -3,6 +3,12 @@
 {
   imports = [ ./. ];
 
+  options = {
+    hardware.nvidia.primeBatterySaverSpecialisation = lib.mkEnableOption "configure a specialisation which turns on NVIDIA Prime battery saver";
+  };
+
+  config = {
+
     hardware.nvidia.prime = {
       offload = {
         enable = lib.mkOverride 990 true;
@@ -10,4 +16,22 @@
       };
       # Hardware should specify the bus ID for intel/nvidia devices
     };
+
+    specialisation = lib.mkIf config.hardware.nvidia.primeBatterySaverSpecialisation {
+      battery-saver.configuration = {
+        system.nixos.tags = ["battery-saver"];
+        imports = [
+          # Leave only the integrated GPU enabled
+          ./disable.nix
+        ];
+        hardware.nvidia = {
+          prime.offload.enable = lib.mkForce false;
+          powerManagement = {
+            enable = lib.mkForce false;
+          	finegrained = lib.mkForce false;
+          };
+        };
+      };
+    };
+  };
 }