etna: chip specs/features handling

- further plumbing to allow switching stuff in the etna driver based on
  chip specifications
- add kernel-independent specs structure to viv
- figure out some bits related to shader state and specs
etna: remove shader test methods from pipe_context

pipe_context now completely matches gallium upstream.
shader test methods have not yet been completely removed (as they're
used by the fb_rawshader demos), but are no longer vtable methods.