gpu: ion: Don't flush secure buffers

Secure buffers have very tight restrictions about mapping into
the non secure world. The buffers are always allocated as uncached
such that there should never be a reason to have a CPU side
non-secure mapping once the buffer is allocated. Some clients may
share the same code path for secure and non-secure transactions.
Unfortunately these clients either cannot or will not distiguish
between secure and non-secure buffers, meaning they will unconditionally
try to flush the buffers. This may lead to bad things. Until clients
manage to properly distinguish between secure and non-secure buffers,
add a check in the cache flushing code to not flush if the buffer is
secure.

Change-Id: Idb0d791a8b74367e0ce0b533f980caedc23044b3
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>