iommu: msm: surround the msm_dma_iommu apis with iommu ifdef

Make all msm_dma_iommu apis depend on CONFIG_IOMMU_API as it is
only used when we have the linux iommu layer available.

Change-Id: I879dc1a9174d498b9b4bc68b2418165f3b2675a3
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>