nvidia: Enable power management

And I was wondering why my system wouldn't suspend properly....

modules/nixos/hardware/gpu/nvidia/default.nix

@@ -11,6 +11,9 @@
       modesetting.enable = true;
       package = config.boot.kernelPackages.nvidiaPackages.beta;
       nvidiaSettings = false;
+      powerManagement = {
+        enable = true;
+      };
     };
   };