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
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