@roykyeah, sorry about that. It started to work for me out of a sudden too. I just couldn't respond here due to 1 post/day limit until now:)
I'm not sure why it started to work, it's the same kernel/dtb version as before. I'll keep watching it on subsequent boots. There's nothing in dmesg about it, just suddenly /dev/cec0 shows up and works fine..
And works in kodi too, out of the box (the one from https://launchpad.net/~liujianfeng1994/+archive/ubuntu/rockchip-multimedia)