drm/ttm: fix incorrect calculate on shrink_pages

shrink_pages is in unit of Order after ttm_page_pool_free,
but it is used by nr_free in next round so need change
it into native page unit

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Roger He <Hongbo.He@amd.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

@@ -455,6 +455,7 @@ ttm_pool_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		freed += (nr_free_pool - shrink_pages) << pool->order;
 		if (freed >= sc->nr_to_scan)
 			break;
+		shrink_pages <<= pool->order;
 	}
 	mutex_unlock(&lock);
 	return freed;