msm: mdss: update the wait logic for sending DSI commands

For video mode panels, the assumption is that the vertical
blanking region will be maximum of 4 ms and hence, we wait for
4 ms before queuing the DSI DMA commands. With the incell panels
having huge vertical blanking region and dynamic fps feature,
this assumption is no longer safe. Update the wait logic by
calculating the vertical blanking duration.

Change-Id: I69fdc182342493a54d78ae3ce5f4729e17452155
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>