dsp: lsm: adjust mutex_lock to avoid deadlock

When dispatch_event in apr and q6lsm_client_free in q6lsm
are running concurrently, there may be deadlock between
session_lock in lsm and svc->m_lock in apr. Adjust session_lock
in q6lsm_client_free to avoid deadlock.

Change-Id: I26eb788d4c17af78272dd1f717717e865dfbd0ac
Signed-off-by: Meng Wang <mengw@codeaurora.org>