iommu/vt-d: Move EIM detection to intel_prepare_irq_remapping

We need this to be detected already when we program the irq
remapping table pointer to hardware.

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