drm/msm/sde: base scaler phase on decimated dimensions

Fix the pipe scaler phase calculations to use the width and
height after decimation is taken into account. This fixes
issues where decimation is required to scale past what is
possible with scaler alone.

Change-Id: I813fae32625ae3e85489ad96540599efcf5a1a29
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>