nixos/nvidia: check modesetting for gdm-wayland only when gdm is enabled

Reported in https://github.com/NixOS/nixpkgs/pull/147153#issuecomment-982695772

nixos/modules/hardware/video/nvidia.nix

···
   in mkIf enabled {
     assertions = [
       {
-        assertion = with config.services.xserver.displayManager; gdm.nvidiaWayland -> cfg.modesetting.enable;
+        assertion = with config.services.xserver.displayManager; (gdm.enable && gdm.nvidiaWayland) -> cfg.modesetting.enable;
         message = "You cannot use wayland with GDM without modesetting enabled for NVIDIA drivers, set `hardware.nvidia.modesetting.enable = true`";
       }