msm_fb: display: add dtv wait for vsync when in blt mode

Currently dtv is waiting for overlay done when in blt mode. When dtv
frame rate is low, this return may be too soon. So before dmae
finishes updating buffer, the buffer may be overwritten by userspace.
Add dtv wait4vsync after overlay done to lock the buffer untill the
next vsync to avoid tearing or flicking.

Change-Id: Ic57544a5305fdf9081bfeeccd8d364395faaf602
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>