i965/rbc: Allocate mcs directly

i965/rbc: Allocate mcs directly

such as we do for compressed msaa. In case of non-compressed simgle
sampled buffers the allocation of mcs is deferred until there is
actually a clear operation that needs the mcs.
In case of render buffer compression the mcs buffer always needed
and there is no real reason to defer the allocation. By doing it
directly allows to drop quite a bit unnecessary complexity.

Patch leaves brw_predraw_set_aux_buffers() a no-op. Subsequent
patches will re-use it and it seemed cleaner to leave it instead
of removing and re-introducing.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>