I'm experiencing the same issue in Ubuntu 14.04 and 15.10 with Radeon HD 6670 under the open source drivers.
Installing the proprietary drivers isn't an option for me, because it brick the entire operating system including the fail-safe terminal. And because I have been an active part in the Ubuntu Quality Assurance team for a long time, I know these drivers are becoming more and more unmaintained in favour of open source ones.
When proprietary drivers used to work it made the desktop slow, and some games stopped working. On the other side, I have no issue with open source drivers in other games apart from Soma.
Here you have a game-play showing the issue:
https://youtu.be/9OwNDHuKa5E
Thanks