drm/i915/bios: limit default outputs to ports A through F

There are two main cases where the default outputs are useful when the
VBT is missing:

- There are some DDI-platform Chromebooks out there that do not have a
  VBT, which worked by coincidence because of the default outputs. The
  machines need to continue to work.

- Early platform enabling when the VBT might not be available. (This
  could be circumvented by using the i915.vbt_firmware parameter.)

Prepare for generating fake child devices for the default outputs by
limiting the number of outputs. We don't want to generate excessive
amounts of fake child devices. This could be perhaps be limited even
more in the future, but match what's possible on all DDI platforms.

Note that limiting the defaults to non-TypeC ports in commit
828ccb31

 ("drm/i915/icl: Add TypeC ports only if VBT is present") is
a more strict limit, and makes this a no-op on recent platforms.

v2: Rewrote commit message

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5c9c9743af1c7265a2c976d582b7a6685ec0c414.1615998927.git.jani.nikula@intel.com