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.