drm/msm/dpu: handle merge_3d configuration in hw_ctl block

Active HW CTL blocks need separate handling for merge_3d flushes.
Implement necessary merge_3d configuration and flushing.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>