drm/i915/vbt: split out defaults that are set when there is no VBT

The main thing are the DDI ports. If there's a VBT that says there are
no outputs, we should trust that, and not have semi-random
defaults. Unfortunately, the defaults have resulted in some Chromebooks
without VBT to rely on this behaviour, so we split out the defaults for
the missing VBT case.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/95c26079ff640d43f53b944f17e9fc356b36daec.1489152288.git.jani.nikula@intel.com