Add nvidia-disable profile

This is already used in XPS 15-9560, but instead of defining it in XPS
15-9560 let's create a separate profile and expose it for those who want
to disable their NVIDIA GPU.

dell/xps/15-9550/default.nix

@@ -4,6 +4,8 @@
   imports = [
     ../../../common/cpu/intel
     ../../../common/pc/laptop
+    # To just use Intel integrated graphics with Intel's open source driver
+    # ../../../common/gpu/nvidia-disable
   ];
 
   # TODO: boot loader
@@ -12,7 +14,4 @@
 
   # This will save you money and possibly your life!
   services.thermald.enable = true;
-
-  # To just use Intel integrated graphics with Intel's open source driver
-  # hardware.nvidiaOptimus.disable = true;
 }