mdp: Remove ion_unmap_iommu calls

There are weird display glitches (and reboot shortly after) when locking/unlocking the device, and a quick look at dmesg indicates the issue to be caused by ion_unmap_iommu being called when nothing has been mapped to iommu. Msm8x60 doesn't use iommu.

Signed-off-by: Sultanxda <sultanxda@gmail.com>