dsp: msm_audio_ion: update logic to pass pointer by reference

Update logic for pointer meant to hold virtual address of ION
buffers allocated, so that the pointer is available across
function boundary.

Change-Id: I7849498b9ccf0cbd7c357fa926eea6c4b65a194f
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>