dma-mapping: use iommu_unmap for unmapping address

Use the iommu_unmap() instead of iommu_unmap_range() for unmapping of
address ranges.

Change-Id: I3f2f7108b21adc2672e7f15ca24667c1c80a9453
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>