msm_fb: display: remove mdp writeback2 overlay kickoff waiting

Before overlay2 kickoff, there is already a dma busy waiting, so once
overlay is kicked off, mdp driver can safely return to userspace. Then
if there is another request while ovlay2 is still busy, dma busy can
take care of it.

Change-Id: Ibb45a8b306a393c8e240fc64b5857e8700afcaba
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>