nixos/xserver: use the requested drivers

This fixes a regression introduced in #482828.

The previous logic was repeatedly looking for a driver named "name" (the
literal identifier), instead of the actual driver.

nixos/modules/services/x11/xserver.nix

@@ -832,7 +832,7 @@ in
     # FIXME: somehow check for unknown driver names.
     services.xserver.drivers = flip concatMap cfg.videoDrivers (
       name:
-      lib.optional (videoDrivers ? name) (
+      lib.optional (videoDrivers ? ${name}) (
         {
           inherit name;
           modules = [ ];