gpu: ion: Pull secure buffers out of Ion

Securing individual buffers does not belong in the Ion framework.
Pull the code out of the framework and into a file which can be
managed separately. The file can be moved elsewhere at a later
date.

Change-Id: Ifaec740abcd78ee1bc3769a6c73014e4309c82be
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>