msm: camera: isp: Fix tasklet and irq handling

Use tasklet kill instead of tasklet disable
to avoid CPU busy waiting.

Besides, mask out all irq mask and do irq disable
to make sure tasklet will not reschedule itself.

Change-Id: Ie980ef0cb183215ccd4f7c7788c6cab083929250
Signed-off-by: Peter Liu <pingchie@codeaurora.org>