Interesting. Ok then, back to square one: no BIOS options to tweak the
integrated graphics memory / controller.

On a side note: is it normal so many error messages in the journal? Like:

kernel: amdgpu 0000:00:01.0: ffff9cccade4d800 pin failed
kernel: [drm:amdgpu_crtc_page_flip_target [amdgpu]] *ERROR* failed to pin new
abo buffer before flip
gdm-Xorg-:0[672]: (WW) AMDGPU(0): flip queue failed: Cannot allocate memory
gdm-Xorg-:0[672]: (WW) AMDGPU(0): Page flip failed: Cannot allocate memory
gdm-Xorg-:0[672]: (EE) AMDGPU(0): present flip failed
...
gdm-Xorg-:0[672]: (WW) AMDGPU(0): get vblank counter failed: Invalid argument

or

kernel: amdgpu: [powerplay] min_core_set_clock not set