intel/aux-map: Factor out some useful helpers

intel/aux-map: Factor out some useful helpers

This breaks add_mapping() into three pieces:

    1. get_aux_entry() adds AUX-TT pages as needed and returns the
       L1 entry index, L1 entry address, and L1 entry map.

    2. gen_aux_map_format_bits_for_isl_surf() computes the format-
       specific information that goes in the AUX-TT entry.

    3. add_mapping() is a lot dumber function that now just adds the
       requested mapping with the requested format bits.

This lets us break out some additional helpers in the API which we want
to use for more direct AUX-TT management in ANV.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3519>