drm/msm/dpu: Drop unused poll_timeout_wr_ptr PINGPONG callback

drm/msm/dpu: Drop unused poll_timeout_wr_ptr PINGPONG callback

This callback was migrated from downstream when DPU1 was first
introduced to mainline, but never used by any component.  Drop it to
save some lines and unnecessary confusion.

Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/534215/
Link: https://lore.kernel.org/r/20230411-dpu-intf-te-v4-12-27ce1a5ab5c6@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>