MFC r273856,273863,273963-273965

- Add two new functions to the AGP driver KPI to bind/unbind arbitrary sets
  of pages into the GTT.
- Avoid possible overflow in agp_generic_alloc_memory.
- In agp(4) avoid the need to flush all cpu caches with wbinvd between
  updating the GTT and flushing the AGP TLB by storing the GTT in
  write-combining memory.
- In agp_amd_bind_page don't flush the AGP TLB.  It's done by the calling
  function.
- agp_generic_unbind_memory: flush AGP TLB before unwiring pages
  agp_bind_pages: assert that pages have been wired down
MFC r273862,273902:

Port the TTM AGP backend to the FreeBSD agp driver and enable AGP support
in the radeonkms driver.

Note: In PCI mode virtual addresses on the graphics card that map to system
RAM are translated to physical addresses by the graphics card itself.   In
AGP mode address translation is done by the AGP chipset so fictitious
addresses appear on the system bus.  For the CPU cache management to work
correctly when the CPU accesses this memory it needs to use the same
fictitious addresses (and let the chipset translate them) instead of using
the physical addresses directly.

Reviewed by:	kib
MFC r273969:

Use default memory type for TTM buffer objects that may be cached.
MFC r269634:

drm: fix usage of vm_phys_fictitious_to_vm_page

vm_phys_fictitious_to_vm_page should not be called directly, even when
operating on a range that has been registered using
vm_phys_fictitious_reg_range. PHYS_TO_VM_PAGE should be used instead
because on arches that use VM_PHYSSEG_DENSE the page might come
directly from vm_page_array.

Reported by:	nwhitehorn
Tested by:	nwhitehorn, David Mackay <davidm.jx8p@gmail.com>
Sponsored by:	Citrix Systems R&D