Slow down, there's a point of confusion here.
An entity in your map, which happens to be a light, is leaking. That's a map geometry issue.
This is not the same as saying that "light is leaking through my displacement", which is a statement of how light-and-shadow are working.
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Displacements never seal the map. You must always put a "real" brush (generally textured with nodraw) to seal your map.
To use a metaphor: Your map is an air-filled space-station in the inky void of space, and displacements are simply bits of mesh netting strung up around the place for your astronauts to block their view and keep them from bumping into things. They are not air-tight and you you need a good solid bulkhead behind them to seal things.
