iommu: msm: Handle unmapping of PTE properly

The unmap api is currently not handling unmapping of page table
entries (PTE) properly. The generic function that calls the msm
unmap API expects the unmap call to unmap as much as possible
and then return the amount that was unmapped.
In addition the unmap function does not support an arbitrary input
length. However, the function that calls the msm unmap function
assumes that this is supported.

Both these issues can cause mappings to not be unmapped which will
cause subsequent mappings to fail because the mapping already exists.

Change-Id: I638d5c38673abe297a701de9b7209c962564e1f1
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>