drm: rework radeon memory map (radeon 1.23)

This code reworks the radeon memory map so it works better
for newer r300 chips and for a lot of older PCI chips.

It really requires a new X driver in order to take advantage of this code.

From: Ben Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>