staging: 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: NTan Xiaojun <tanxiaojun@huawei.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/staging/vboxvideo/vbox_ttm.c

@@ -234,7 +234,6 @@ static struct ttm_bo_driver vbox_bo_driver = {
 	.verify_access = vbox_bo_verify_access,
 	.io_mem_reserve = &vbox_ttm_io_mem_reserve,
 	.io_mem_free = &vbox_ttm_io_mem_free,
-	.io_mem_pfn = ttm_bo_default_io_mem_pfn,
 };
 
 int vbox_mm_init(struct vbox_private *vbox)