drm/komeda: Add slave pipeline support

One crtc can use two komeda_pipeline, and one works as master and as
slave. the slave pipeline doesn't have its own output and timing
ctrlr, but pre-composite the input layer data flow and then feed the
result to master. the pipeline configuration like:

slave-layer-0 \
...            slave->CU
slave-layer-4 /         \
                        \
master-layer-0 --------> master->CU -> ...
 ...                  /
master-layer-4 ------>

Since komeda Compiz doesn't output alpha, so the slave->CU result
only can be used as bottom input when blend it with master input data
flows.

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