i965/miptree: Create a hiz mcs type

i965/miptree: Create a hiz mcs type

This seems counter to the goal of consolidating hiz, mcs, and later ccs buffers.
Unfortunately, hiz on gen6 is a thing the code supports, and this wart will be
helpful to achieve that. Overall, I believe it does help unify AUX buffers on
gen7+.

I updated the size field which I introduced in the previous patch, even though
we have no use for it.

XXX: As I mentioned in the last patch, the height given to the MCS buffer
allocation in intel_miptree_alloc_mcs() looks wrong, but I don't claim to fully
understand how the MCS buffer is laid out.

v2: rebase on master (Lionel)

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