i965/gen7+: Allocate MCS buffers for use by fast color clears.
!UPSTREAM: i965/gen7+: Support fast color clear values other than (0,0,0,0)

TODO:

- Test

- Move into earlier patches
!UPSTREAM: Temporary debugging hack to dump clear color bits
!UPSTREAM: Temporary debugging hack to print out contents of MCS buffer
!UPSTREAM: Temporary debugging hack to test RT resolves
!UPSTREAM: Stop doing pre-emptive resolves