drm/i915/bios: refactor DDC pin and AUX CH sanitize functions

Add separate functions to get the port by DDC pin and AUX channel.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a8b4afc08dd03e08c1403531e7f5ab33d777b1db.1559308269.git.jani.nikula@intel.com