iommu: msm_iommu_sec: use dma_free_attrs to match dma_alloc_attrs

Currently we're using dma_free_coherent to free a DMA allocation that
was allocated with dma_alloc_attrs.  DMA allocation and free should
always use matching attributes.  Failure to do so could result in
instabilities.  Fix this by using dma_free_attrs.  Also fix an erroneous
comment regarding the dma_alloc_attrs piece.

Change-Id: I99fed0d27b14895a5a9099944a25ef9ee1072614
Suggested-by: Joseph Chang <joechang@qti.qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>