Seems like you are using an old kernel - though what I call old may be current
for what's released.

The hanging for me stopped with 4.4, but 4.3 is release.

If you are using 4.3 maybe booting with the option -

amdgpu.enable_scheduler=1

will help.

If you are used to compiling your own kernels then using one from -

http://cgit.freedesktop.org/~agd5f/linux/

like drm-next-4.4 should be stable.

Or if you want to test/use the new powerplay try amdgpu-powerplay