The code to determine the pre-ICL DSI port control register is
repeated several times. Consolidate.

vlv_dsi_clear_device_ready() is left with the open-coded version
due to the weirdness with port A vs. C on VLV/CHV.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231101114212.9345-6-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>