iommu: msm: Code cleanup

Clean up the code by using a built-in function instead of re-writing
the same functionality. This will make the code cleaner and
reduce the amount of code that needs to be maintained leading
to more reliable code.

Change-Id: I6751b1043f6bc978ac073494cef8442af18f4fd7
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>