i965: Rename various gen6 #defines to match the documentation.

i965: Rename various gen6 #defines to match the documentation.

This should make it easier to cross-reference the code and hardware
documentation, as well as clear up any confusion on whether constants
like CMD_3D_WM_STATE mean WM_STATE (pre-gen6) or 3DSTATE_WM (gen6+).

This does not rename any pre-gen6 defines.