I installed ATI5000Controller kext, I installed ATI5000Injector kext, I modified AMDRadeonX3000 and AMD5000Controller kext with vendor/revision ID 0x68F91002 and I confirm that's a Cedar personality. The best result I had was that the card was recognized as a Radeon HD 5450 but with just 5 Mb of vram and no QE/CI support. I tried to remove ATI kext letting just the AMD ones like it was with the original DP1 build. Same problem.
I have to mention that It worked perfectly with the original DP1 build, the problem occurs with the new seed proposed via Software update. It seems that Apple changed something with the graphics drivers as I noticed that the ATI kext disappered after the update.
I'm still looking for a way to solve the issue but I really don't know what's going wrong.
I've attached all the kext used in this post if it can help someone to fix this out
Good luck !