msm: ais: isp: Handling buffer use after getting it freed

In the code, start_fetch can try to access the
buffer pointer variable after free, as the
same pointer can be freed at RELEASE_BUF call too
at the same time. Hence fixing this race condition.

Change-Id: Ifb643bace27064e1324d714aebed706b48e44b65
Signed-off-by: Rahul Sharma <sharah@codeaurora.org>