drm/msm/sde: program pixel extension from scalar config from sde plane

Avoid implicit programming of pixel extension and scalar blocks
of an SSPP by programming them from sde plane.

Change-Id: Iac324e7d75f29ebf4b3d8bd2edc38b62c98d5193
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>