gpu: ion: only secure buffers when supported

There are some targets where securing Ion buffers is not supported
through the TZ API used in the MSM secure buffer code. Currently, we
simply try the allocation and if it fails we let the clients secure it
themselves through some other means. However, due to a recent change to
the TZ error logging in the kernel, the current "ask for forgiveness"
approach results in logspam. Fix this by moving to an "ask for
permission" approach: if the TZ API to secure the buffers is not
available, don't attempt to use it.

Change-Id: Ie003d5139a0622ce9c18719ca2697525e23e7733
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>