msm_fb: display: empty pending queue during suspend

Overlay_play() adds a pipe into pending queue and pan_display()
will commit those pipes to hardware and empty queue. This patch
will make sure queue is empty at suspend so that no unexpected
pipes will be commited to hardware by pan_display() at resume.
Otherwise, iommu page fault may happen.

Change-Id: Iafdf469c6cdbf5d469c9dee114555a8d1adb9c66
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>