ion: Fix ion compilation errors on Kona

Include the stub version of the function
msm_dma_unmap_sg_attrs() for successful ION
compilation when LAZY MAPPING is not defined.
Perform the clean up of msm_dma_unmap_sg()
since it is not called/used anywhere.

Change-Id: Ib56e4ae66aba3b3b14229771c6f5ea85007f1829
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>