i965/gen7+: Set up MCS in SURFACE_STATE whenever MCS is present.

On Gen7+, MCS buffers are used both for compressed multisampled color
buffers and for "fast clear" of single-sampled color buffers.

Previous to this patch series, we didn't support fast clear, so we
only used MCS with multisampled bolor buffers.

As a first step to implementing fast clears, this patch modifies the
code that sets up SURFACE_STATE so that it configures the MCS buffer
whenever it is present, regardless of whether we are multisampling or
not.
i965/gen7+: Create helper functions for single-sample MCS buffers.
!UPSTREAM: i965/gen7+: Allocate MCS buffers for use by fast color clears.

TODO:

- 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