iommu/irq_remapping: Consolidate irq domain lookup

Now that the iommu implementations handle the X86_*_GET_PARENT_DOMAIN
types, consolidate the two getter functions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20200826112331.741909337@linutronix.de