msm: vidc: Avoid deadlock in dcvs during scale clocks

There is a deadlock condition in dcvs when driver
tries to queue a buffer which it held, on receipt of
release reference event from the firmware. The driver
tries to acquire the registeredbufs.lock, which it has
already acquired in the same thread in handle_event_change().
Avoid this condition by removing the need to acquire the
lock, by computing the number of buffers held in the driver
whenever the buffer references are incremented\decremented.

Change-Id: Ic985f0672adc1e2c9e8a37f3d72883ef234d1b72
Signed-off-by: Arun Menon <avmenon@codeaurora.org>