media: venus: helpers: Lock outside of buffer queue helper

After adding more logic in vdec buf_queue vb2 op it is not
practical to have two lock/unlock for one decoder buf_queue.
So move the instance lock in encoder and decoder vb2 buf_queue
operations.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>