msm: camera: isp: Reduce burst count

Decrement the burst count for dual stream in case of dual vfe. The
count is tracked in both streams and hence should be reduced for
both streams once buffer is done from both vfe.

Change-Id: I681390c2b3a2a0c4861721204ac19302335bf542
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>