drm/xe/mocs: Bring comment about mocs back to reality

The mocs documentation was copied from i915 and doesn't match the
reality in xe. Reword it so it matches what the code is doing.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20231117174049.527192-2-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>