msm: vidc: access vb2 buffer under vb2_queue lock

Access vb2 buffer under vb2_queue->lock and if streaming
only else vb2 buffer might be removed by vb2 framework
while response handler thread is accessing it which results
in video usecases failure.

Change-Id: Ie4fbade47e2e4db282e8714125e7aad8712b2028
Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>