Sorry, but I don't know.  I don't follow or use DPH, I just know how the GHC repo works, and that they had a problem with including additional library sub-repos which just happened to include (probably out of date, since DPH is developed separately) DPH libraries.  As such, I would not expect DPH to be included in the official 7.4 release unless the DPH folks submit an updated version (and probably not even then since GHC is in the release candidates phase, so it's a bit too late to add a newer DPH into it).