https://bugs.freedesktop.org/show_bug.cgi?id=93652
--- Comment #5 from Nicolai Hähnle nhaehnle@gmail.com --- Thanks for the reports.
Those backtraces are typical consequences of a GPU hang and are unfortunately not helpful for isolating the root cause.
One thing you could try is starting Steam with R600_DEBUG=nodcc from a terminal window.
Also, if it doesn't take too long for the hang to occur (Wolfgang mentions a few minutes), you could try recording an apitrace, and see whether you also get a lockup when you replay the trace. Such a trace would be very helpful.