drm/msm/sde: don't dirty scaler properties if null

Avoid always flagging scaler configuration properties if
the user space keeps setting it to a null value. These
properties are meant to support incoming user pointers
rather than traditional drm blob properties, so avoid
recalculating the plane programming if they are set to
a null value more than once.

CRs-Fixed: 2062083
Change-Id: I88b7a3065670e7fa8827c91dbe856b428127de1b
Signed-off-by: Clarence Ip <cip@codeaurora.org>