iommu/vt-d: Disable IRQ remapping in intel_prepare_irq_remapping

Move it to this function for now, so that the copy routines
for irq remapping take no effect yet.

Tested-by: ZhenHua Li <zhen-hual@hp.com>
Tested-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>