drm/msm/dpu: use struct dpu_hw_merge_3d in dpu_hw_pingpong

Use struct dpu_hw_merge_3d pointer in struct dpu_hw_pingpong rather
than using struct dpu_hw_blk. This is the only user of dpu_hw_blk.id,
which will be cleaned in the next patch.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20210515190909.1809050-4-dmitry.baryshkov@linaro.org


Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>