msm: kgsl: For IOMMU only decide to flush when memory is unmapped

In case of IOMMU memory needs to only be flushed when it is
unmapped and not mappped. Also, when an application is running
the number of unmaps would in most cases be less than number of
maps, so this way the number of flushes wil be fewer.

Change-Id: Id4107631c343045edbc744125035202844405910
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>