1. Add scaler to writeback pipeline to enable the writeback scaling support
2. Display HW can not do upscaling for writeback, check it when validate.

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>