VT-d: install sync_cache hook on demand

VT-d: install sync_cache hook on demand

Instead of checking inside the hook whether any non-coherent IOMMUs are
present, simply install the hook only when this is the case.

To prove that there are no other references to the now dynamically
updated ops structure (and hence that its updating happens early
enough), make it static and rename it at the same time.

Note that this change implies that sync_cache() shouldn't be called
directly unless there are unusual circumstances, like is the case in
alloc_pgtable_maddr(), which gets invoked too early for iommu_ops to
be set already (and therefore we also need to be careful there to
avoid accessing vtd_ops later on, as it lives in .init).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>