drm/i915/dsc: move rc_buf_thresh values to common helper

drm/i915/dsc: move rc_buf_thresh values to common helper

The rc_buf_thresh values are common to all DSC implementations. Move
them to the common helper together with the code to propagate them to
the drm_dsc_config.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230517102807.2181589-3-dmitry.baryshkov@linaro.org
Acked-by: Dave Airlie <airlied@redhat.com>