Fix some minor TSO issues:
- Improve description of TSO limits.
- Remove a not needed KASSERT()
- Remove some not needed variable casts.

Sponsored by:	Mellanox Technologies
Discussed with:	lstewart @
MFC after:	1 week
On 965 and higher, map GTT as write-combining.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
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
Fix gcc build: preserve const qualifier when casting input values.

Noticed by:	bz@
Submitted by:	dim@
Update comment.

Noted by:	dim
Approved by:	secteam (des)
MFC after:	4 days
Add workaround for vt efifb's early use of PHYS_TO_DMAP

In vt_efifb_init the framebuffer's physaddr is passed to PHYS_TO_DMAP
before the DMAP is setup. The result is not actually accessed until
after the mapping is setup, though. Loosen the assertion in PHYS_TO_DMAP
for now, to allow use when dmaplimit == 0.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D1142
Test errno against EEXIST as well.

MFC after:	3 days
Remove myself from bxe(4) maintenance.