drm/msm/dpu: use PINGPONG_NONE to unbind WB from PP

drm/msm/dpu: use PINGPONG_NONE to unbind WB from PP

Currently the driver passes the PINGPONG index to
dpu_hw_wb_ops::bind_pingpong_blk() callback and uses separate boolean
flag to tell whether WB should be bound or unbound. Simplify this by
passing PINGPONG_NONE in case of unbinding and drop the flag completely.

Suggested-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/540969/
Link: https://lore.kernel.org/r/20230604031308.894274-2-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>