dsp: msm_audio_ion: unmap dma only if unmap kernel succeeds

If buf unmap is executed without successful kernel unmap
the vmap counter mismatches. Check for error scenarios in
kernel unmap function and ignore buf unmap if not necessary.

Change-Id: I3c5835dcdfe05db4baaa5cf904995ed8f89e9cfe
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>