Compile tested only.

This was a deeper change than I was hoping for, due to the
layering of the pipe_texture implementation in each driver on
top of a shared pipe_buffer implementation in the shared code.

Have modified the shared code to act as a set of convenience
routines operating on nouveau_bo objects.

Each driver now uses the u_resource_vtbl technique to split the
implementation of pipe_resources between the existing miptree code
for textures and a new, minimal buffer implementation in each
driver.

Eventually these should be combined, not least because APIs are now
allowing things like binding buffer resources as textures and render
targets.