msm: vidc: Protect queued buffers list with lock

Queued buffers list can be accessed across different
threads so it should be protected with a lock. Without
proper locking the lists might get corrupted resulting
in frame drops and crash eventually.

CRs-fixed: 400702

Change-Id: Ie316b6744db342203e23a83bf6194983ea5eecca
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>