drm/i915: Extract icl_combo_phy_loadgen_select()

Pull the convoluted loadgen calculation into a small helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211006204937.30774-7-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>