drm/ttm: remove set but not used variable 'driver'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/ttm/ttm_execbuf_util.c: In function 'ttm_eu_fence_buffer_objects':
drivers/gpu/drm/ttm/ttm_execbuf_util.c:190:24: warning:
 variable 'driver' set but not used [-Wunused-but-set-variable]

It not used any more after
commit f2c24b8 ("drm/ttm: flip the switch, and convert to dma_fence")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/ttm/ttm_execbuf_util.c

@@ -187,14 +187,12 @@ void ttm_eu_fence_buffer_objects(struct ww_acquire_ctx *ticket,
 	struct ttm_buffer_object *bo;
 	struct ttm_bo_global *glob;
 	struct ttm_bo_device *bdev;
-	struct ttm_bo_driver *driver;
 
 	if (list_empty(list))
 		return;
 
 	bo = list_first_entry(list, struct ttm_validate_buffer, head)->bo;
 	bdev = bo->bdev;
-	driver = bdev->driver;
 	glob = bo->bdev->glob;
 
 	spin_lock(&glob->lru_lock);