drm/radeon/kms: allocate vram scratch page on 6xx+

The vram scratch was originally only used on some 7xx asics
to work around a hw bug.  Allocate the scratch page on all 6xx+
radeons and set the MC_VM_SYSTEM_APERTURE_DEFAULT_ADDR to point
to it.  We shouldn't ever hit it since we limit the system
aperture to vram or vram and AGP, but better safe than sorry.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>