intel-iommu: Move duplicate list-mangling code into unlink_domain_info() helper

Now we have four copies of this code, Linus "suggested" it was about time
we stopped copying it and turned it into a helper.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>