agp_intel.c provides support for AGP on Intel chipsets from 440/BX to
875.  This intersects with the agp_i810.c, which supports all Intels
from i810 to Core i5/7.  Both agp_intel.c and agp_i810.c are compiled
into kernel when device agp is specified in config, and agp_i810
attach seems to be selected by chance due to linking order.

Strip support for 810 and later from agp_intel.c.  Since 440-class
chipsets do not support any long-mode capable CPUs, remove agp_intel.c
from amd64 kernel file list.  Note that agp_intel.c is not compiled
into agp.ko on amd64 already.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks