nixos/xserver: Use null as default for dpi option
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 <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs