ASoC: msm: Fix the possilibity of writing to already released buffer

Calling msm_pcm_playback_close right after trigger start/resume might
release the buffer that is later used by the event handler to process
Q6 RUN-CMD response. The patch makes sure the event handler only uses
the buffer when the pcm driver is not closed.

Change-Id: I8b3acf39c41a4419a5a4f8023f797e003feabcb7
CRs-Fixed: 320179
Signed-off-by: Jay Wang <jaywang@codeaurora.org>