Based on some BIOS configuration (GGC register in host bridge, bit 1),
IGP may declare subclass as either VGA-compatible, or non-VGA.  The
difference is that in the later case, IGP does not claim VGA cycles.
Other than that, the device functions normally, and agp_i810 should
attach to it.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Initial attachment of the agp(4) to Haswell IGP.  There is no handling
of cacheablility control bits in GTT PTEs yet.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week