drm/msm/dpu: Use fixed DRM DSC helper for det_thresh_flatness

drm/msm/dpu: Use fixed DRM DSC helper for det_thresh_flatness

The current dpu_hw_dsc calculation for det_thresh_flatness does not
match the downstream calculation or the DSC spec.

Use the DRM DSC helper for det_thresh_flatness to match downstream
implementation and the DSC spec.

Fixes: c110cfd1753e ("drm/msm/disp/dpu1: Add support for DSC")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/539275/
Link: https://lore.kernel.org/r/20230329-rfc-msm-dsc-helper-v14-6-bafc7be95691@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>