drm/msm/sde: issue flush for rot fetch update only if differs

Current rotator fetch update always flushes the timing engine and
this is not required. Add check to flush the timing engine only
if rotator fetch update is different from last update.

Change-Id: I7acdd7787e5ee56285be92c29da4f6ee5cfb8aff
Signed-off-by: Alan Kwong <akwong@codeaurora.org>