drivers: staging: ion: mark secure page private

check_valid_vmid does not mark page private when
releasing the page to secure pool. Do it.

Change-Id: I258561421c520a7b8b50dbf9421bc0aea459dfee
Fixes: cae595f4 ("ion: Check for valid vmids for msm system heap")
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>