Hi Michael, the patch you provided did indeed get rid of all errors, but now
modprobe enters an infinite loop on the test that previously failed

[   62.294510] [drm] ring test on 0 succeeded in 0 usecs
[   62.294578] [drm] ring test on 3 succeeded in 1 usecs
[   62.294901] [drm:evergreen_irq_set], evergreen_irq_set: sw int gfx
[   62.294943] [drm] ib test on ring 0 succeeded in 0 usecs
[   62.294989] [drm:evergreen_irq_set], r600_irq_set: sw int dma
[   62.721958] [drm:evergreen_irq_set], r600_irq_set: sw int dma
[   62.731965] [drm:evergreen_irq_set], r600_irq_set: sw int dma
[   62.741979] [drm:evergreen_irq_set], r600_irq_set: sw int dma
...
[  434.061917] [drm:evergreen_irq_set], r600_irq_set: sw int dma

Just saw your last comment and so I'm off to change remove FENCE_SWAP_ENABLE. 
Thanks