i965/miptree: Use enum instead of boolean.

i965/miptree: Use enum instead of boolean.

ISL_AUX_USAGE_NONE happens to be the same as "false", but let's do the
right thing and use the enum.

v2: fix intel_miptree_finish_depth too (Caio)

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>