drm/ttm: make the device list mutex static

This way it can protect the whole BO global state.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

include/drm/ttm/ttm_bo_driver.h

@@ -407,7 +407,6 @@ struct ttm_bo_global {
 	struct kobject kobj;
 	struct ttm_mem_global *mem_glob;
 	struct page *dummy_read_page;
-	struct mutex device_list_mutex;
 	spinlock_t lru_lock;
 
 	/**