drm/amdgpu: Use list_del_init in amdgpu_mn_unregister

Otherwise bo->shadow_list (which is aliased by bo->mn_list) will not
appear empty in amdgpu_ttm_bo_destroy and cause an oops when freeing
former userptr BOs.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -359,7 +359,7 @@ void amdgpu_mn_unregister(struct amdgpu_bo *bo)
 	head = bo->mn_list.next;
 
 	bo->mn = NULL;
-	list_del(&bo->mn_list);
+	list_del_init(&bo->mn_list);
 
 	if (list_empty(head)) {
 		struct amdgpu_mn_node *node;