drm/radeon/kms: Fix chremap setup on RV770 CE

CE variant requires a different chremap setup.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=35472



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>