I now have spoken to VIA Embedded on Facebook, asking about new drivers that support HD playback on the P820 board and the answed I got was:
VIA Embedded yes, we do but we don’t provide chipset driver directly to end users. If you are our customer, please contact with our sales representatives.
You many go to this web site, http://linux.via.com.tw/support/downloadFiles.action
We don’t wanna release it since there are so many Linux version and it’s not quite easy for us to maintain.