drm/nouveau: remove the default io_mem_pfn set

The default interface situation has been taken into the framework, so
remove the default set of each module.
Signed-off-by: Tan Xiaojun <tanxiaojun@huawei.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -1672,5 +1672,4 @@ struct ttm_bo_driver nouveau_bo_driver = {
 	.fault_reserve_notify = &nouveau_ttm_fault_reserve_notify,
 	.io_mem_reserve = &nouveau_ttm_io_mem_reserve,
 	.io_mem_free = &nouveau_ttm_io_mem_free,
-	.io_mem_pfn = ttm_bo_default_io_mem_pfn,
 };