LinuxKPI: Add video/vga.h header

Sponsored by: Serenity Cyber Security, LLC
Obtained from: OpenBSD
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42793

(cherry picked from commit 5e6bddfc43300f344d44afdc80f1bb7a37bd84fc)