drm/i915/mtl: Add missing steering table terminators

The termination entries were missing for a couple of the recently-added
MTL steering tables.

Fixes: f32898c9 ("drm/i915/xelpg: Add multicast steering")
Fixes: a7ec65fc

 ("drm/i915/xelpmp: Add multicast steering for media GT")
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221028224022.964997-1-matthew.d.roper@intel.com