msm_fb: display: free previous iommu buffer only at overlay_unset

Since wait4dmap is used at pipe_commit, there is possibility that
iommu buffer had been freed before vsync. Iommu page fault will
happen under this situation. Therefore only previous played iommu
buffer can be freed at overlay_unset. However all iommu buffers
are freed at suspend.

CRs-fixed: 440254
Change-Id: I9c3b77de255b74fd608aa28bb0f2277a58e2ba97
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>