msm: mdss: add support for variable refresh rate for 8916/8939

Flush the MDP and DSI timing registers during variable refresh rate
operation for 8916/8939. Also, there is no need to wait for vsync
interrupt before programming the timing registers due to their
double buffer nature.

Change-Id: Ia349729069bd49f0c40ce41cccefdfb67cafe44c
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>