Don't serialise map data writes on D3D12 if we've baked to cached buffer
Don't serialise map data writes on D3D12 if we've baked to cached buffer

* When we have a cached buffer that already has the map contents, reserialising
  the byte buffer is pointless as we don't use it for anything.
* The serialise involves allocating a buffer, memcpy'ing it, then freeing it
  which can add up for large map writes.

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