drm/ttm: check the return value of kzalloc

In the function ttm_page_alloc_init, kzalloc call is made for variable
_manager, we need to check its return value, it may return NULL.
Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/ttm/ttm_page_alloc.c

@@ -1007,6 +1007,8 @@ int ttm_page_alloc_init(struct ttm_mem_global *glob, unsigned max_pages)
 	pr_info("Initializing pool allocator\n");
 
 	_manager = kzalloc(sizeof(*_manager), GFP_KERNEL);
+	if (!_manager)
+		return -ENOMEM;
 
 	ttm_page_pool_init_locked(&_manager->wc_pool, GFP_HIGHUSER, "wc", 0);