drm/msm/dpu: drop the DPU_PINGPONG_TE flag

The DPU_PINGPONG_TE flag became unused, we can drop it now.

Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/555542/
Link: https://lore.kernel.org/r/20230904020454.2945667-4-dmitry.baryshkov@linaro.org