ion: Skip zeroing on secure buffers

Secure buffers are never passed to userspace and are controlled
by the secure world so there is no real need to zero. Pass the
dma attribute to skip zeroing.

Change-Id: Iad870d0d7732d3dea09443418b9294cb9e05b5e0
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>