vt: add option to ignore NO_VGA flag in ACPI

To workaround buggy firmware that sets this flag when there's actually
a VGA present.

Reported and tested by: Yasuhiro KIMURA <yasu@utahime.org>
Sponsored by: Citrix Systems R&D
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D16003