drm/msm/sde: update inline prefill programming

Rotator start programming for inline prefill doesn't take
effect on the next frame, but rather all subsequent frames
after the next one. However, prefill bandwidth calculations
are based on the requirements for the next frame, so the
calculated prefill bandwidth and rotator start programming
can be mismatched whenever the rotator start is adjusted.

This patch avoids this issue by fixing the rotator start
to a constant value, and adjusting the prefill bandwidth
for each frame instead.

Change-Id: I5410f63856248a035d627126962dd407f01a8f0d
Signed-off-by: Clarence Ip <cip@codeaurora.org>
[cohens@codeaurora.org: Resolved trivial merge conflict]
Signed-off-by: Steve Cohen <cohens@codeaurora.org>