This couldn't be split because it would break bisecting.

Summary:
* r300g,r600g: stop using u_vbuf
* r300g,r600g: also report that the FIXED vertex type is unsupported
* u_vbuf: refactor for use in the state tracker
* cso: wire up u_vbuf with cso_context
* st/mesa: conditionally install u_vbuf