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
can be freed at RELEASE_BUF call too at the same time.
Hence fixing this race condition.

Change-Id: Iba53a0910d672d6e7d1fc5bad3e9fadeb3b59df8
Signed-off-by: Suprith Malligere Shankaregowda <supgow@codeaurora.org>