iommu/vt-d: Consolidate irq domain getter

The irq domain request mode is now indicated in irq_alloc_info::type.

Consolidate the two getter functions into one.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20200826112331.530546013@linutronix.de