vchiq_arm: Add completion records under the mutex

An issue was observed when flushing openmax components
which generate a large number of messages returning
buffers to host.

We occasionally found a duplicate message from 16
messages prior, resulting in a buffer returned twice.

While only one thread adds completions, without the
mutex you don't get the protection of the automatic
memory barrier you get with synchronisation objects.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>