drm/radeon/kms: Fix caching mode selection for GTT object

GTT object can either be cached,uncached or wc just let core ttm
pick the best mode according to how the bo driver and GTT memory
type was initialized.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>