nixos/displaylink: write out required xorg.conf fragment

Invoke xrandr to actually connect the device.

Additionally, we let systemd create the logs directory and use our module loader
instead of handling it manually.

nixos/modules/hardware/video/displaylink.nix

@@ -19,6 +19,21 @@ in
   config = mkIf enabled {
 
     boot.extraModulePackages = [ evdi ];
+    boot.kernelModules = [ "evdi" ];
+
+    environment.etc."X11/xorg.conf.d/40-displaylink.conf".text = ''
+      Section "OutputClass"
+        Identifier  "DisplayLink"
+        MatchDriver "evdi"
+        Driver      "modesetting"
+        Option      "AccelMethod" "none"
+      EndSection
+    '';
+
+    # make the device available
+    services.xserver.displayManager.sessionCommands = ''
+      ${lib.getBin pkgs.xorg.xrandr}/bin/xrandr --setprovideroutputsource 1 0
+    '';
 
     # Those are taken from displaylink-installer.sh and from Arch Linux AUR package.
 
@@ -47,18 +62,13 @@ in
       description = "DisplayLink Manager Service";
       after = [ "display-manager.service" ];
       conflicts = [ "getty@tty7.service" ];
-      path = [ pkgs.kmod ];
 
       serviceConfig = {
         ExecStart = "${displaylink}/bin/DisplayLinkManager";
         Restart = "always";
         RestartSec = 5;
+        LogsDirectory = "displaylink";
       };
-
-      preStart = ''
-        mkdir -p /var/log/displaylink
-        modprobe evdi
-      '';
     };
 
   };