disp: msm: sde: fix vblank wait after cache read mode update

This change fixes the vblank wait after system cache read mode
update. Without this change the wait does not happen since there is
no pending kickoff. This change uses encoder api to flush the
configuration and explicitly waits for vblank.

Change-Id: I8942f9b638e784c8fd9b5df33a9ccc7087a5eaef
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>