Maybe one of the phone devs can mention what debug flags the etnaviv-gpu driver supports? Oh, duh… If you can put an external keyboard in, run alt-sysrq-9 to up the debugging information. Then when a freeze happens run alt-sysrq-l, alt-sysrq-m, alt-sysrq-p, alt-sysrq-q, alt-sysrq-t, and alt-sysrq-w. Those should dump a large amount of information about what the computer is doing when it freezes.
If you don’t have a keyboard, or the USB subsystem stops working, you can also have a script echo those commands (the last letter) into /proc/sysrq-trigger. Something like echo 9 > /proc/sysrq-trigger to turn on debugging.