intel/isl: Move get_format_encoding function to isl

intel/isl: Move get_format_encoding function to isl

Move get_format_encoding function to isl and rename to
isl_get_aux_map_format_encoding.

v2:
- Rename isl_get_aux_map_format_encoding to
  isl_format_get_aux_map_encoding (Jason Ekstrand)

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Suggested-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3719>