This finally fixes the radeon memory mapping bug that was incorrectly
fixed by the previous patch.  This time, we use the actual vram size as
the size to calculate how far to move the AGP aperture from the
framebuffer in card's memory space.

If there are still issues with this patch, they are due to bugs in the X
driver that I'm working on fixing too.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Mark M. Hoffman <mhoffman@lightlink.com>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>