drm/radeon/kms: never treat rs4xx as AGP

RS4xx+ IGP chips use an internal gart, however,
some of them have the agp cap bits set in their pci
configs.  Make sure to clear the AGP flag as AGP will
not work with them.

Should fix fdo bug 27225

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