drm/msm/dpu: Introduce PINGPONG_NONE to disconnect DSC from PINGPONG

drm/msm/dpu: Introduce PINGPONG_NONE to disconnect DSC from PINGPONG

Disabling the crossbar mux between DSC and PINGPONG currently
requires a bogus enum dpu_pingpong value to be passed when calling
dsc_bind_pingpong_blk() with enable=false, even though the register
value written is independent of the current PINGPONG block.  Replace
that `bool enable` parameter with a new PINGPONG_NONE dpu_pingpong
flag that triggers the write of the "special" 0xF "crossbar
disabled" value to the register instead.

Changes in v4:
-- more details to commit text

Changes in v5:
-- rewording commit text suggested by Marijn
-- add DRM_DEBUG_KMS for DSC unbinding case

Changes in v8:
-- fix checkpatch warning

Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/539505/
Link: https://lore.kernel.org/r/1685036458-22683-6-git-send-email-quic_khsieh@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>