drm/radeon/kms: Check if bo we got from ttm are radeon object or not

If they are not radeon object don't do anythings special for them,
this avoid rare oops than can happen in a complex use case.

[airlied: additional fixups]

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