drm/i915/bios: document child device config dvo_port values a bit better

Move closer to child device config struct while at it. No functional
changes.

Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
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/abbad7b4a3f0c35d48844cc0950af3606e9a3142.1503600621.git.jani.nikula@intel.com