dsp: msm_audio_ion: store device pointer locally for all cases

Device pointer needs to be stored during probe of msm_audio_ion
platform driver so that the same can be used later for mapping
dma_buff allocated for different clients. Store device pointer
for both cases where SMMU may be present or absent on a particular
platform.

Change-Id: I3d3d4ad254d41ac9490ff0dc98b95ffbc3e5042d
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>