Hi, before this commit MCLK works ok, reverting did the job.  
[1] drm/amdgpu: refine uvd 6.0 clock gate feature
https://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-next-4.10-wip&id=1b7eab1f8346ab3b8e4fc54882306340a84497a8

There is two stages for this issue:
[1] MCLK is HIGH (maybe more power consumption)
[2] MCLK is LOW - performance hit.

[1]for idle (two displays but only one is active)
cat /sys/class/drm/card0/device/pp_dpm_sclk 
0: 300Mhz *
1: 466Mhz 
2: 751Mhz 
3: 1019Mhz 
4: 1074Mhz 
5: 1126Mhz 
6: 1169Mhz 
7: 1260Mhz
cat /sys/class/drm/card0/device/pp_dpm_mclk 
0: 300Mhz 
1: 2000Mhz *

[2]drm/amdgpu:impl vgt_flush for VI(V5)
https://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-next-4.10-wip&id=ddfe1db18752b08d88d81cb7b661e1f982fc5d04
MCLK is set to LOWEST state (300MHz) and nothing can change that until revert
[1].