Same problem.
My graphics cards:
lspci | grep VGA
01:05.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI RS880M
[Mobility Radeon HD 4225/4250]
02:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Park
[Mobility Radeon HD 5430/5450/5470]
If radeon.runpm=0 all o.k.
If manually turning off the dGPU using switcheroo then freeze

Use Ubuntu 14.04 with 13.rc1