Registering an output for a non-existent port (on a given SKU) can lead
to problems when trying to use the port, for instance timeouts during
power well enabling. Since there are no strap bits for port detection we
have to rely on VBT for this, so do that here.

There are no known SKUs where any of the A-E ports are non-existent, so
to reduce the likelihood of breakage due to incorrect VBT information,
do this detection only for port F (which is known to be missing on some
ICL SKUs).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108915
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220132604.25222-2-imre.deak@intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>