drm/rockchip: remove redundant statement

The call to arm_iommu_detach_device() on the previous line sets
dev->archdata.mapping to NULL so this call is always a no-op.
Signed-off-by: NJohn Keeping <john@metanate.com>

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -244,7 +244,6 @@ static int rockchip_drm_unload(struct drm_device *drm_dev)
 	drm_kms_helper_poll_fini(drm_dev);
 	component_unbind_all(dev, drm_dev);
 	arm_iommu_detach_device(dev);
-	arm_iommu_release_mapping(dev->archdata.mapping);
 	drm_mode_config_cleanup(drm_dev);
 	drm_dev->dev_private = NULL;