input: qcom-hv-haptics: add mutex lock for FIFO play

When stop command comes during FIFO refill, it would free custom_effect
FIFO sample memory and set the pointer to NULL, and this would cause
the NULL pointer dereference during FIFO refill. To fix this, add a
mutex lock between FIFO stop and FIFO refill sequence to avoid a race.

Change-Id: I8792c2183f06e84fca3a7c4dddfed667b25f2528
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>