You can download 1.2.5 from https://www.plex.tv/downloads/
Right, this is interesting. I tried a nightly build of 1.3.0 that I found the link to whilst looking into a separate problem I'm having with PMP on a RPi3 not having any 5.1 audio since 1.3.0.
I put the 1.3.0 (generic) nightly build on the Atom box. As soon as it booted in, I tried a video and HW decoding worked. I then went through and tweaked all the settings (to get sound working, etc) and tried to play the same video and HW decoding did not work, for any videos subsequently.
So I reflashed the 1.3.0 nightly, and HW decoding worked again. This time I went through changing the settings one by one and test playing a video each time to see if HW decoding still worked.
Sure enough, HW decoding breaks after I enable Display mode switching. It seems to permanently break it too, as even if I subsequently disable display mode switching, HW decoding still does not work for any video. Only a reboot 'fixes' it.
I then allowed the automatic update to take it to 1.3.1, which is exactly the same. Enabling display mode switching breaks HW decoding, until display mode switching is disabled and the machine rebooted.