msm_fb: display: wait4vsync after timing generator is turned off

When continuous splash is enabled, timing generator is enabled at
bootloader, it then needs to be turned off at mdp4_dsi_video_on()
to switch displayed content from frame buffer. wait4vsync after
timing generator is turned off is necessary to make sure DMAP has
finished fetching splash logo from original splash buffer. Otherwise,
DMAP iommu page fault may happen.

Change-Id: I35f9fb9da76f93b60d4719d147aea3246c770c9f
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>