HDMI screen not detected with PRIME

[elTed]

Hi everyone,
I recently bought a laptop with AMD 4800H + RX5600M and it has a triple boot, with win10, gentoo ( gentoo-sources 5.9.8 ) and manjaro ( kernel 5.6) .
In both linux systems, switcheroo is compiled in kernel.

• Under windows, the discrete GPU is activated automatically when I insert the HDMI cable or I launch a 3D software; otherwise is off.
  
• Under manjaro, the discrete GPU is always on "DynPwr" (probably because of HDMI-AUDIO); HDMI is detected and functioning.
  
• Under gentoo, the discrete GPU is off until I use DRI_PRIME=1 (that's ok), but HDMI monitor is never detected, neither by kscreen nor by xrandr.
  

Strangely, sometime during shutdown the screen output briefly shows up on the external monitor!
If I export DRI_PRIME=1, then the external monitor goes white with some colored pixel.

The desired behavior is the same as windows, but just getting the HDMI output is enough.

I already read the wiki page about AMDGPU and PRIME offloading on gentoo and manjaro, plus usual google search, but I can't get it done.
I compared the .config and the options for amdgpu and fbdev are the same, as far I can see.
In dmesg, the only "suspect" messages are:

• amdgpu: RAS: optional ras ta ucode is not available
  
• [drm] Cannot find any crtc or sizes
  
• [drm] Failed to add display topology, DTM TA is not initialized.
  

The last two seem to be the most concerning.

Does anyone have idea of how to correct this or point me in the right direction?
Thanks