msm_fb: display: free writeback buffer at correct time

When mdp pipe is freed at end of video play back, Writeback
buffer is freed while mdp still fetchs data from it. This cause
system crashed and blue screen shows on screen. This patch
will free writeback buffer at correct time.

Change-Id: I46a5031c13d86f943d6604713e1841b482dffe96
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>