gpu: ipu-v3: initially clear all interrupts

If we want to stop resetting the IPU in the future, masking all
interrupts before registering the irq handlers will not be enough to
avoid spurious interrupts. We also have to clear them.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Liu Ying <gnuiyl@gmail.com>