iommu: Fix compilation errors when !CONFIG_QCOM_LAZY_MAPPING

Definition of msm_dma_unmap_sg_attrs is not present when
CONFIG_QCOM_LAZY_MAPPING is not selected this would result 
in compilation errors, fix this by adding the definition.

Change-Id: Ie4e92463a1641a002e99c72b325e224b79066936
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>