drm/komeda: Attach scaler to drm as private object

According to the komeda pipeline configuration, attach scaler to drm as
private object.

v2: Rebase

Signed-off-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>