CPU usage/threading on agd5f 4.15-wip

[Andy Furniss]

On 4.15-wip with patches that make powerplay work I have an issue with 
one game = unreal tournament alpha.

This doesn't seem to affect other things like xonotic or unigine demos.

Issue = on fixes 4.14 and other older kernels I've tried, the game is OK 
ish perf wise, 20-40 fps.

On 4.15-wip its 10fps. Using HUD one obvious difference is that the slow 
case is only mostly using one cpu core, on a good kernel all 4 of my old 
phenom 2 x4 are used.

Both kernels seem based on 4.13-rc5 so I guess it's something amdgpu.

Trouble is, I can't bisect 4.15-wip, going back near or > 200 commits I 
lock soon after startx.

So is there any change devs can think of that would cause this?