!UPSTREAM: i965: Allocate MCS buffers for use by fast color clears.

TODO:

- modify callers of gen7_wm_surface_state to set up the MCS buffer
  when present, even if not multisampling.

- write blorp code to do fast color clears

- write blorp code to do color resolves

- write code to ensure that color resolves happen when necessary

- write code to do fast rather than slow color clears when possible