msm: vidc: Add mutex locks for buffer reference update

buf_ref_put() and buf_ref_get() which update the output
buffer reference counts were not protected by mutex locks.
This could lead to a race condition, whereby when these functions
act on a buffer node in registered list, it would have been freed by
another thread, causing a kernel crash. Protecting the buffer
reference updates with mutex lock overcomes the race condition.

Change-Id: I4ea4ba19b6af14e0db565921276b82bb64eef16e
Signed-off-by: Arun Menon <avmenon@codeaurora.org>