msm: vidc: fix corruption of registered buffer list

The buffer list for output buffers is accessed from
different threads leading to race conditions. Consider
the case of firmware throwing up a fatal error, which gets
propagated to v4l2 client. The v4l2 client closes the
driver fd, which will trigger msm_vidc_release_buffers
to release all output buffers and free node from
the registered buffer list. During this time, if
firmware sends a release reference event, the buffer
node being accessed in msm_vidc_release_buffers could
be freed, triggering kernel panic.

Change-Id: I522edff06a62dbc2ab60d6cca5990babb18d5c6c
Signed-off-by: Arun Menon <avmenon@codeaurora.org>