Revert radeon AGP aperture offset changes

This reverts the series of commits

	67dbb4ea
	281ab031
	47807ce3

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>