msm: vidc: Use MM heap for prediction buffer during secure encode

We are incorrectly using the firmware heap to allocate prediction
buffers. This violates an agreement between TZ and HLOS that only
the firmware will reside in the FW heap. As such, move the buffer
to MM heap.

Change-Id: I2c91499c38043abfa21ae3443d3a8f22d6474eaa
CRs-fixed: 380161
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>