iommu/ipmmu-vmsa: Simplify group allocation

We go through quite the merry dance in order to find masters behind the
same IPMMU instance, so that we can ensure they are grouped together.
None of which is really necessary, since the master's private data
already points to the particular IPMMU it is associated with, and that
IPMMU instance data is the perfect place to keep track of a per-instance
group directly.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>