nouveau: fix disp disabling with GSP

This func ptr here is normally static allocation, but gsp r535
uses a dynamic pointer, so we need to handle that better.

This fixes a crash with GSP when you use config=disp=0 to avoid
disp problems.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231222043308.3090089-4-airlied@gmail.com

drivers/gpu/drm/nouveau/nvkm/engine/disp/base.c

@@ -209,7 +209,7 @@ nvkm_disp_dtor(struct nvkm_engine *engine)
 		nvkm_head_del(&head);
 	}
 
-	if (disp->func->dtor)
+	if (disp->func && disp->func->dtor)
 		disp->func->dtor(disp);
 
 	return data;
@@ -243,8 +243,10 @@ nvkm_disp_new_(const struct nvkm_disp_func *func, struct nvkm_device *device,
 	spin_lock_init(&disp->client.lock);
 
 	ret = nvkm_engine_ctor(&nvkm_disp, device, type, inst, true, &disp->engine);
-	if (ret)
+	if (ret) {
+		disp->func = NULL;
 		return ret;
+	}
 
 	if (func->super) {
 		disp->super.wq = create_singlethread_workqueue("nvkm-disp");