i965: Stop considering if msrt aux buffers need aux buffer

i965: Stop considering if msrt aux buffers need aux buffer

Auxiliary buffers are always created with sample number of zero
which effectively prevents intel_miptree_create_layout() from trying
to associate auxiliary buffers with auxiliary buffers.

Now that there is more direct path available lets start using it
instead and stop even checking for such (im)possibility.

v2 (Ben): Do not signal msaa layout with explicit argument but
          using layout_flags instead.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>