thinkpad: use lib.mkDefault on all unmergable options

lenovo/thinkpad/t440p/default.nix

@@ -1,10 +1,10 @@
-{ config, pkgs, ... }:
+{ config, lib, pkgs, ... }:
 
 {
   imports = [ ../. ];
 
   boot = {
-    extraModprobeConfig = ''
+    extraModprobeConfig = lib.mkDefault ''
       options bbswitch use_acpi_to_detect_card_state=1
     '';
     kernelModules = [ "tpm-rng" ];