anv,blorp,iris: Set MOCS for COMPUTE_WALKER post-sync operation

We don't current enable post sync operations, but it is probably
better to set them to "internal" MOCS than to remove the non-zero
checking for this genxml field.

Reworks:
 * Fix COMPUTE_WALKER in cmd_buffer_trace_rays (s-b Jason)

Fixes: 7b78b2fcac6 ("intel/genxml: Assert that all MOCS fields are non-zero on Gfx7+")
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13624>