intel/dev: Set max_wm_threads to 0 in the Gfx9+ devinfo structs

intel_device_info_init_common calculates this for Gfx9+ based on
max_threads_per_psd and slice information.  Mark it as zero in the
structures to make clear that the value there isn't useful, and make
it easier to diff binaries for the next commit's refactors.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <mesa/mesa!33764>