ASoC: msm-cpe-lsm: Synchronize LAB thread start and stop

The sequence of commands from userspace cannot be assumed. Irrespective
of the sequence of IOCTL's from userspace, the driver needs to handle
the starting and stopping of the LAB (Look-Ahead Bufferring) gracefully
to avoid unexpected synchronization issues. Fix to synchronize
start/stop of LAB thread and LAB buffer allocation so that LAB thread
never accesses the LAB buffer when the buffer is de-allocated.

CRs-fixed: 764869
Change-Id: I7f484ab4450c29a1262fcc14d51d3446a1e16c39
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>