drm/radeon/kms: remove rs4xx gart limit

We used to limit the rs4xx gart aperture to 32 MB, but I suspect
that was due to not meeting the alignment requirements of the
aperture.  This patch should only be applied after:
"drm/radeon/kms: fix gtt MC base alignment on rs4xx/rs690/rs740 asics"
has been applied.

This patch should probably soak for a bit in d-r-t.

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