iommu: msm: use list_for_each_entry_safe for deletion

Use list_for_each_entry_safe to safely delete mappings while
iterating over the mappings list.

CRs-Fixed: 1067012
Change-Id: I5326ee8e58d291b1d9b07649b87632d7e9102e0d
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>