tee: don't assign shm id for private shms

mainline inclusion
from mainline-v5.7-rc1
commit f1bbaced
category: cleanup
bugzilla: NA
CVE: NA

Prepare for fixing CVE-2021-44733.
--------------------------------

Private shared memory object must not be referenced from user space. To
guarantee that, don't assign an id to shared memory objects which are
driver private.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>