drm/msm/sde: add null checks to scaler3 lut setup

Plane state is set to NULL during color fill operations.
This patch adds checks to gracefully handle NULL plane
state during the scaler3 setup path.

CRs-Fixed: 2019301
Change-Id: I3ac5bd8f26e68afe559bf7c815da904392d3de13
Signed-off-by: Clarence Ip <cip@codeaurora.org>