iommu/arm-smmu: fix incorrect comment regarding TLB invalidation

Commit 1463fe44 ("iommu/arm-smmu: Don't use VMIDs for stage-1
translations") moved our TLB invalidation from context creation time to
context destruction time, but forgot to update an associated comment.

This patch fixes the broken comment.

Signed-off-by: Will Deacon <will.deacon@arm.com>