ion: Forbid multi-VMID allocation requests for the secure system heap

The secure system heap does not support multi-VMID allocations,
so do not allow them. Also, fortify the secure allocation check
for the system heap allocation path.

Change-Id: I51d53274c2374632db778461ba7505de4d723439
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>