i965: Wrap MCS miptree in intel_miptree_aux_buffer

i965: Wrap MCS miptree in intel_miptree_aux_buffer

This will allow us to treat HiZ and MCS the same when using as an
auxiliary surface buffer.

v2: (Ben) Minor rebase conflict resolution.
   Rename mcs_buf to aux_buf to address upcoming change for hiz specific buffers.
   That second thing is essentially a squash of:
   i965/gen8: Use intel_miptree_aux_buffer for auxiliary buffer - which didn't need
   to be separate in my opinion.

v3: rebase on master (Lionel)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com> (v1)
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>a (v2)
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> (v3)
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>