nixos/xserver: Use null as default for dpi option

Commit 98d9bba introduced this option as a nullOr type and it actually
checks whether null has been set and only appends -dpi if that's the
case. So let's actually set the default to null instead of 0.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/services/x11/xserver.nix

@@ -221,7 +221,7 @@ in
 
       dpi = mkOption {
         type = types.nullOr types.int;
-        default = 0;
+        default = null;
         description = "DPI resolution to use for X server.";
       };