i965: Add and use a single miptree aux_buf field

i965: Add and use a single miptree aux_buf field

We want to add and use a function that accesses the auxiliary buffer's
clear_color_bo and doesn't care if it has an MCS or HiZ buffer
specifically.

v2 (Jason Ekstrand):
* Drop intel_miptree_get_aux_buffer().
* Mention CCS in the aux_buf field.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com> (v1)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>