msm: camera: isp: avoid race on free buffer queue

In addition to the protection of buffer in buffer queue,
buffer queue data struct itself could have race condition
when deinit buffer manager and get buf happen in the same
time.

Move the buffer queue lock to protect the buffer queue itself.

Change-Id: I50a4c58c22f1e568bd1f57ad5a9ea1123aca495b
Signed-off-by: Peter Liu <pingchie@codeaurora.org>