staging: drm/imx: use generic irq chip unused field to block out invalid irqs

This depends on the patch "genirq: irqchip: Add mask to block out invalid irqs"
and masks out reserved bits using the unused interrupt bitfield.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>