drm/amdgpu: fix tear down order in amdgpu_vm_pt_free

When freeing PD/PT with shadows it can happen that the shadow
destruction races with detaching the PD/PT from the VM causing a NULL
pointer dereference in the invalidation code.

Fix this by detaching the the PD/PT from the VM first and then
freeing the shadow instead.
Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: https://gitlab.freedesktop.org/drm/amd/-/issues/2867
Cc: <stable@vger.kernel.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm_pt.c

@@ -642,13 +642,14 @@ static void amdgpu_vm_pt_free(struct amdgpu_vm_bo_base *entry)
 
 	if (!entry->bo)
 		return;
+
+	entry->bo->vm_bo = NULL;
 	shadow = amdgpu_bo_shadowed(entry->bo);
 	if (shadow) {
 		ttm_bo_set_bulk_move(&shadow->tbo, NULL);
 		amdgpu_bo_unref(&shadow);
 	}
 	ttm_bo_set_bulk_move(&entry->bo->tbo, NULL);
-	entry->bo->vm_bo = NULL;
 
 	spin_lock(&entry->vm->status_lock);
 	list_del(&entry->vm_status);