drm/msm/dpu: enable writeback on SM8550

Enable WB2 hardware block, enabling writeback support on this platform.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/562328/


Signed-off-by: Rob Clark <robdclark@chromium.org>