ENGR00273500 IPUv3 dev: use wait_event_interruptible for ipu task

The ipu task thread checks outstanding tasks to be done on waiting
event uninterruptibly on the function find_task()'s return value.
However, sleeping on waiting event uninterruptibly contributes to
system load average value. This patch changes wait_event() to
wait_event_interruptible() to avoid the load average value inflation.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>