drm/ttm: fix missing NULL check in the new page pool

The pool parameter can be NULL if we free through the shrinker.
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Martin Peres <martin.peres@mupuf.org>
Acked-by: Martin Peres <martin.peres@mupuf.org>
Reported-by: Andy Lavr <andy.lavr@gmail.com>
Tested-by: Andy Lavr <andy.lavr@gmail.com>
Link: https://patchwork.freedesktop.org/patch/399365/

drivers/gpu/drm/ttm/ttm_pool.c

@@ -135,7 +135,7 @@ static void ttm_pool_free_page(struct ttm_pool *pool, enum ttm_caching caching,
 		set_pages_wb(p, 1 << order);
 #endif
 
-	if (!pool->use_dma_alloc) {
+	if (!pool || !pool->use_dma_alloc) {
 		__free_pages(p, order);
 		return;
 	}