tonga mclk regression

[Tom St Denis]

I've bisected it to this commit

[root at fx6 linux2]# git bisect bad
df1e63942063a0638a5813e5904988d834db2665 is the first bad commit
commit df1e63942063a0638a5813e5904988d834db2665
Author: Rex Zhu <Rex.Zhu at amd.com>
Date:   Fri Sep 1 13:46:20 2017 +0800

     drm/amd/powerplay: delete eventmgr layer in poweprlay

     Reviewed-by: Alex Deucher <alexander.deucher at amd.com>
     Signed-off-by: Rex Zhu <Rex.Zhu at amd.com>
     Signed-off-by: Alex Deucher <alexander.deucher at amd.com>

:040000 040000 5b955ab18919974b351c81565044f3280253fe03 
35f848a30a0b57d7fdee53133ae5d8e066406d8f M      drivers

The commit before this has a properly throttling mclk with 
glmark2/xonotic-glx and this commit does not (locks at 300MHz).  Should 
point that the commit ahead of this locks at 150MHz (lowest setting).

I haven't actually looked at this commit for a fix yet.

Cheers,
Tom