drm/radeon/kms: fix up gart setup on rs600

Set up rs600 gart like r600:
- set gart system aperture to vram
- inside gart system aperture is unmapped*
- outside gart system aperture is mapped*

*mapped refers to memory handled by page tables

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