iommu/vt-d: Add helper to flush caches for context change

This helper is used to flush the related caches following a change in a
context table entry that was previously present. The VT-d specification
provides guidance for such invalidations in section 6.5.3.3.

This helper replaces the existing open code in the code paths where a
present context entry is being torn down.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240701112317.94022-2-baolu.lu@linux.intel.com
Link: https://lore.kernel.org/r/20240702130839.108139-7-baolu.lu@linux.intel.com


Signed-off-by: Will Deacon <will@kernel.org>