GPU: ipu: fix lockup caused by pending chained interrupts

Even with the oops fixed by a previous patch, the system still fails to
kexec, due to a stuck chained interrupt locking the system.  We must
disable the child interrupts prior to setting up the irq chip to ensure
we don't get stuck here.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>