Revert radeon AGP aperture offset changes

This reverts the series of commits

	67dbb4ea33731415fe09c62149a34f472719ac1d
	281ab031a8c9e5b593142eb4ec59a87faae8676a
	47807ce381acc34a7ffee2b42e35e96c0f322e52

that changed the GART VM start offset.  It fixed some machines, but
seems to continually interact badly with some X versions.

Quoth Ben Herrenschmidt:

  "So I think at this point, the best is that we keep the old bogus code
   that at least is consistent with the bug in the server. I'm working on a
   big patch to X that reworks the memory map stuff completely and fixes
   those issues on the server side, I'll do a DRM patch matching this X fix
   as well so that the memory map is only ever set in one place and with
   what I hope is a correct algorithm..."

Signed-off-by: Linus Torvalds <torvalds@osdl.org>