msm: camera: isp: Use scratch buffer instead of invalid address

Use a scratch buffer instead of progarmming an invalid address to
write master when buffer is unavailable or the stream is not
going to output any additional buffers.

Change-Id: I526170f582211a7283e3763cf35f8846950c514b
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>