Writeback split is also for workaround the size limitation of d71 scaler.
Like layer_split, writeback downscaling also can use two scalers to handle
the scaling half-by-half. The only differnence is writback needs a
standalone component (splitter)'s help to split the composition result.
The data pipeline of writeback split as below:

                   /-> scaler-0 ->\
 compiz -> splitter                merger -> wb_layer -> memory
                   \-> scaler-1 ->/

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>