msm_fb: display: fix iommu page fault when iommu buffer freed

There has possibility that pipe commit come very close to next vsync.
This may cause two consecutive pipe_commits happen within same vsync
period which cause iommu page fault when previous iommu buffer is
freed since mdp stil fetching it. Set ION_IOMMU_UNMAP_DELAYED flag
at ion_map_iommu() to add delay un mapping iommu buffer to fix this problem.
Otherwise four buffer mechanism is needed to avoid this problem..
Also ion_unmap_iommu() may take long time to free an ion buffer. Move
mdp4_overlay_iommu_unmap_freelist(mixer) called after stage_commit()
to increase chance to have pipe_commit (up to stage_commit) is completed
within same vsync period
This patch also fix an potential race condition at wait_for_completion
between show_event thread and wait4vsync thread.

Change-Id: I673b0e368bbcd3120bc26569fe7a7bb82931daaa
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
(cherry picked from commit ada63be0fd32e729e4b91aeeea2a5cfe0830148c)
(cherry picked from commit 799796f15e0b78ffe47975db1cf7f3791cb98129)