I've to rectify my last affirmation about the patch. 

I've patched the 4.18.5 kernel, compiled and rebooted the guest without
rebooting the host.
The guest vm was already in a bad state (300 MHz (MCLK)). Using a pathed kernel
in a guest vm in such state did not solve the situation.

After this, i tried resetting the host.
The first start of the host has led to normal behavior. Resetting the guest
without resetting the host seems to maintain the correct behavior (1750 MHz
(MCLK)).

The patch seems to work only if the card is in a "good" state. if, for some
reasons, the card tunrns in a bad state, the patch cannot solve the problem.

I also notice that the message "powerplay ini fails" is gone, but now i get:

[   36.352542] amdgpu: [powerplay] 
                last message was failed ret is 0
[   36.353460] amdgpu: [powerplay] 
                last message was failed ret is 0
[   36.353468] amdgpu: [powerplay] 
                failed to send message 260 ret is 255 
[   36.353471] amdgpu: [powerplay] 
                failed to send message 145 ret is 255 
[   36.353475] amdgpu: [powerplay] 
                last message was failed ret is 255