drm/msm/sde: avoid dereferencing scaler blob if null state

Add checks to avoid dereferencing the user scaler blob if
the plane state is null, which is possible if the scaler
configuration is requested internally.

Change-Id: I497cdd8e2d35afa842451321eee98ae3b9c34bf2
Signed-off-by: Clarence Ip <cip@codeaurora.org>