msm: mdss: Wait for vsync before iommu attach during cont splash

During first borderfill commit in splash cleanup, wait for vsync
before iommu attach to prevent IOMMU page faults during bootup.

Change-Id: I68b7162d090513b082cbd4c0013731cedc0ebdbe
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>