msm: isp: Fix list corruption

List corruption can happen due to stale entries in the list. Also, list
corruption can happen if we add the same buffer twice to the shared list.

With this fix,
 1. Whenever we get a flush call, we clear all elements in the
     shared list.
 2. Changed the list_del_init api to list_del.
 3. Added bufq lock protection to msm_isp_update_put_buf_cnt

Change-Id: I0ea4bd8bf706c3871e2f280f80f6e70c37d9eb36
Signed-off-by: Nagesh Reddy <nageshsreddy@codeaurora.org>
Signed-off-by: Peter Liu <pingchie@codeaurora.org>