The VBT contains the DDC pin to use for specific ports. Alas, sometimes
the field appears to contain bogus data, and while we check for it later
on in intel_gmbus_get_adapter() we fail to check the returned NULL on
errors. Oops results.
The simplest approach seems to be to catch and ignore the bogus DDC pins
already at the VBT parsing phase, reverting to fixed per port default
pins. This doesn't guarantee display working, but at least it prevents
the oops. And we continue to be fuzzed by VBT.
One affected machine is Dell Latitude 5590 where a BIOS upgrade added
invalid DDC pins.