i965: Prepare to delete intel_miptree_alloc_ccs()

i965: Prepare to delete intel_miptree_alloc_ccs()

We're going to delete intel_miptree_alloc_ccs() in the next commit. With
that in mind, replace the use of this function in
do_single_blorp_clear() with intel_miptree_alloc_aux() and move the
delayed allocation logic to it's callers.

v2: Duplicate the delayed allocation comment (Topi Pohjolainen).

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