msm: mdss: Wait for one vsync after turn off TG

TG control register is double buffered register. When
disable, it will only take effect once vsync has come.
So wait for at least once vsync to ensure TG is off.

Change-Id: I35d8c8be4bc72ababd2a04bb7718688404d775eb
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>