drm/radeon/kms: R3XX/R4XX AGP asic use PCI GART not PCIE GART

R3XX/R4XX AGP asic use the old PCI GART block, not the new PCIE GART.
Make sure we pick the right GART when disabling AGP.

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