drm/msm/dpu: Configure CWB in writeback encoder

Cache the CWB block mask in the DPU virtual encoder and configure CWB
according to the CWB block mask within the writeback phys encoder

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/637501/
Link: https://lore.kernel.org/r/20250214-concurrent-wb-v6-8-a44c293cf422@quicinc.com


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