nouveau: NV04/NV10/NV20 PGRAPH engtab functions
NV04/NV10 load_context()/save_context() are stubs. I don't know enough about
how they work to implement them sanely. The "old" context_switch() code
remains hooked up, so it shouldn't break anything.
NV20 will probably break if load_context() works. No inital context values
are filled in, so when the first channel is created PGRAPH will probably end
up having its state zeroed. Some setup from nv20_graph_init() will probably
need to be moved to the per-channel context setup.