gpu: ion: disallow mmap'ing secure buffers

There is a requirement to disallow mapping secure buffers to
userspace. The reason for this requirement is that once some memory
has been locked down by the secure environment, the application
processor is not allowed to access that memory. Fix the CP heap to
follow this policy.

CRs-Fixed: 442914
Change-Id: Ie7a6d82be58574c8579d8d46ec82551c88b81900
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>