drm/i915/dpio: Clean up VLV/CHV DPIO PHY register defines

The DPIO PHY registers follow clear numbering rules. Express
those in a few macros to get rid of the hand calculated
final offsets.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240422083457.23815-13-ville.syrjala@linux.intel.com


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