iommu/vt-d: Don't disable IR when it was previously enabled

Keep it enabled in kdump kernel to guarantee interrupt
delivery.

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