iommu/amd: Enforce alignment for MSI IRQs

Make use of the new alignment capability of
alloc_irq_index() to enforce IRQ index alignment
for MSI.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Fixes: 2b324506 ('iommu/amd: Add routines to manage irq remapping tables')
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>