slim-msm.c: Check for iommu dev before dma_free_coherent

Add a check for iommu device handler before doing dma_free_coherent
as a part of SSR clean up.
This fixes the kernel page table corruption happening due to wrong
device handler used during freeing the resources during SSR cleanup.
This change ensures we use same dev handle for allocating and
freeing the DMA memory.

Change-Id: Ic42d42446dd9954f9c1fc7d282b03d1fa4bc4980
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>