msm: camera: isp: Put the VFE irq payload at a single point

Put the VFE irq event payload back on the free list at the end of the
tasklet, when we are sure we are done with it. This makes sure that
we do not read a payload that is already put back on the free list.

Change-Id: I531bd778ee9d04d3d4f79ee991c470601989cc57
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>