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.