I also have those freezes in the Steam context menu when using DRI3 and
DRI_PRIME=1 to start Steam. Never with the integrated GPU or with fglrx.

I have a muxless AMD/AMD hybrid 6480G/6650M setup, I'm running Ubuntu
16.04/Unity on kernel 4.6-rc6 with radeon.runpm=0 set as boot (DRI_PRIME
doesn't work with dpm enabled for me), and this happens with mesa 11.2 and
11.3-devel (oibaf or padoka ppa).

I must add that I can play games perfectly without any freezes, it's just when
navigating in Steam context menus or settings that I encounter this, and the
system is unresponsive, no Ctrl-alt-fx to tty or such.