llvmpipe: check buffers in llvmpipe_is_resource_referenced.

llvmpipe: check buffers in llvmpipe_is_resource_referenced.

Now that buffers can be used as textures or render targets
make sure they aren't skipped.

Fix suggested by Jose Fonseca.

v2: added a couple of assertions so we can actually guarantee
we check the resources and don't skip them. Also added some comments
that this is actually a lie due to the way the opengl buffer api works.