IOMMU: properly check whether interrupt remapping is enabled

IOMMU: properly check whether interrupt remapping is enabled

... rather than the IOMMU as a whole.

That in turn required to make sure iommu_intremap gets properly
cleared when the respective initialization fails (or isn't being
done at all).

Along with making sure interrupt remapping doesn't get inconsistently
enabled on some IOMMUs and not on others in the VT-d code, this in turn
allowed quite a bit of cleanup on the VT-d side (if desired, that
cleanup could of course be broken out into a separate patch).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: "Zhang, Xiantao" <xiantao.zhang@intel.com>