amdgpu: fix performance drop

[Christian König]

Hi Sinclair,

you probably have missed this set of patches I've send to the dri-devel 
mailing list a while ago.

It basically replaces some work for TTM I've did back in April last year 
and you reviewed with a different implementation.

The original callback design turned out to have to much overhead which 
made this change necessary. Could you take a look at the new 
implementation as well?

Thanks in advance,
Christian.