iommu/amd: Don't register interrupt remapping irqdomain when IR is disabled

Registering the remapping irq domain unconditionally is potentially
allowing I/O-APIC and MSI interrupts to be parented in the IOMMU IR domain
even when IR is disabled. Don't do that.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201111144322.1659970-1-dwmw2@infradead.org