Looks like agd5f found the problem with RS780 - needed to set the bus type in the X driver. The fix has been pushed to the 6xx-7xx branch of radeonhd. With the latest radeonhd code (and drm) you should now be able to start up the DRI interface, get (slow) EXA acceleration, and have Xv albeit with a blank-ish image.

Tomorrow we look into why Xv isn't working