nixos: Split mesa setup from xserver.nix

With kmscon, it is now possible to have a system without X that still
needs the mesa setup in /run/opengl-driver

Signed-off-by: Shea Levy <shea@shealevy.com>

nixos/modules/module-list.nix

@@ -233,6 +233,7 @@
   ./services/x11/hardware/multitouch.nix
   ./services/x11/hardware/synaptics.nix
   ./services/x11/hardware/wacom.nix
+  ./services/x11/mesa.nix
   ./services/x11/window-managers/awesome.nix
   #./services/x11/window-managers/compiz.nix
   ./services/x11/window-managers/default.nix