drm/i915: Introduce rkl_get_combo_buf_trans()

Give RKL its own get_buf_trans() func.

v2: Drop the FIXME since the spec was clarified to
    indicate that TGL values are used for the HDMI/eDP cases.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608073603.2408-12-ville.syrjala@linux.intel.com