drm/ast: remove unused driver_private access

gem_bo->driver_private is never read by ast nor DRM core. No need to set
it. Besides, drm core clears it during setup, anyway.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ast/ast_ttm.c

@@ -321,7 +321,6 @@ int ast_bo_create(struct drm_device *dev, int size, int align,
 		return ret;
 	}
 
-	astbo->gem.driver_private = NULL;
 	astbo->bo.bdev = &ast->ttm.bdev;
 
 	ast_ttm_placement(astbo, TTM_PL_FLAG_VRAM | TTM_PL_FLAG_SYSTEM);