drm/radeon/kms: stop putting VRAM at 0 in MC space on r600s.

The Lenovo W500 laptop hangs inside an SMI on brightness changes,
I thought it just needed the VGA disable but it turned out to require
slightly more work, setting the MC locations up just like the IGP
chip requirements seems to make it all happy again and I can boot
and play with brightness.

We should probably just do this for all chips and give up the VRAM
at 0x0 idea, it never seems to buy us anything but pain.

Signed-off-by: Dave Airlie <airlied@redhat.com>