drm/msm/dpu: Skip trigger flush and start for CWB

For concurrent writeback, the real time encoder is responsible for
trigger flush and trigger start. Return early for trigger start and
trigger flush for the concurrent writeback encoders.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/637505/
Link: https://lore.kernel.org/r/20250214-concurrent-wb-v6-12-a44c293cf422@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>