drm/i915/bios: split ddi port parsing and debug printing

Split ddi port parsing and debug printing to clarify the functional
parts of parse_ddi_port(), which are quite small nowadays.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/34e0dd92b7f7e9076df1f01b542347e599ec6653.1654870175.git.jani.nikula@intel.com