I know that the xvba-vaapi bridge had not be updated since last June (per GIT Log in freedesktop.org). Anyone can comment whether you are currently still using it? In what kind of application and GPU are you using?
With Chrome development choosing vaapi as the video acceleration layer someone from AMD should really take this up and make the UVD2 hardware useful in Linux. They could almost immediately gain traction with the large number of applications already supporting vaapi.