disp: msm: sde: add delay after porches changed

This change adds 2 vsync delay after changing the timing engine
parameters namely the porches to update the refresh rate.
This ensures that panel vsync is updated as per new timing before
modifying it again.

Change-Id: I5866ea2f6f2e68bc8ce7435c4a5dbe27d8ebdd91
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>