drm/display/dsc: Add flatness and initial scale value calculations

drm/display/dsc: Add flatness and initial scale value calculations

Add helpers to calculate det_thresh_flatness and initial_scale_value as
these calculations are defined within the DSC spec.

Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/539282/
Link: https://lore.kernel.org/r/20230329-rfc-msm-dsc-helper-v14-1-bafc7be95691@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>