drm/msm/sde: allow forcing of kernel scaler calculation

This patch adds a debugfs entry to the planes to force the driver
to ignore any custom scaler configuration from the user space.

CRs-Fixed: 2019305
Change-Id: I98596a1aaa0629ca1bfe81ab5c01a0d7854859e3
Signed-off-by: Clarence Ip <cip@codeaurora.org>