msm_fb: display: add mutex on overlay_play to fix iommu page fault

An iommu page fault may heppen When a pipe_commit is executed
in between of frame dropped and pipe_queue during overlay play.
An mutex is necessary to mutual exclusive between pipe_commit
and overlay_play.

Change-Id: I736e821c6c648a6ebd435303a1906a40e9b75791
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>