(In reply to Alex Deucher from comment #7)
> Check your xorg log and make sure acceleration is enabled.

It seems that acceleration disables itself after the first startx attempt locks
up the GPU:

[   326.027] (--) RADEON(0): Chipset: "SUMO2" (ChipID = 0x9644)
[   326.027] (II) RADEON(0): GPU accel disabled or not working, using shadowfb
for KMS
[   326.027] (II) Loading sub module "shadow"
[   326.027] (II) LoadModule: "shadow"
[   326.027] (II) Loading /usr/lib/xorg/modules/libshadow.so
[   326.047] (II) Module shadow: vendor="X.Org Foundation"
[   326.047]     compiled for 1.18.0, module version = 1.1.0
[   326.047]     ABI class: X.Org ANSI C Emulation, version 0.4
[   326.047] (II) RADEON(0): KMS Color Tiling: disabled
[   326.047] (II) RADEON(0): KMS Color Tiling 2D: disabled
[...]
[   326.247] (WW) RADEON(0): Direct rendering disabled
[   326.247] (II) RADEON(0): Acceleration disabled
[...]
[   326.252] (II) AIGLX: Screen 0 is not DRI2 capable
[   326.252] (EE) AIGLX: reverting to software rendering
[   326.329] (II) AIGLX: enabled GLX_MESA_copy_sub_buffer
[   326.331] (II) AIGLX: Loaded and initialized swrast
[   326.331] (II) GLX: Initialized DRISWRAST GL provider for screen 0

The full log is in this attachment:
https://bugs.freedesktop.org/attachment.cgi?id=121448

I also uploaded another xorg log from the first attempt that causes the lockup,
in case you want to see the difference between the two or something.