drm/radeon/kms: fix RS600 MC setup.

Again we try to put VRAM at 0, and it didn't work on this chipset,
reports of corrupt RAM appeared on irc and bugzilla.

Fix the vram location according to what the BIOS setup, I'm not 100%
sure we don't need the same thing on rs690/rs780/rs880, we probably
should do it there just in case as its what the DDX does.

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