common: Move Intel generation-specific config from cpu to gpu

These modules only included gpu configuration, so belong in that
location.

common/gpu/intel/sandy-bridge/default.nix

@@ -0,0 +1,9 @@
+{
+  imports = [ ../. ];
+
+  # Enables RC6, RC6p and RC6pp.
+  # Last two are only available on Sandy Bridge CPUs (circa 2011).
+  boot.kernelParams = [
+    "i915.enable_rc6=7"
+  ];
+}