intel-iommu: Performance improvement for dma_pte_clear_range()

It's a bit silly to repeatedly call domain_flush_cache() for each PTE
individually, as we clear it. Instead, batch them up and flush a whole
range at a time. We might as well refrain from recalculating the PTE
address from scratch each time round the loop too.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>