Hello
Thank you for the answers and happy not to have raised a bug report!
@Rich : intel_iommu=off alone, still an error in dmesg (code -5) but display Ok, low resolution
@Curage : vmalloc=256M alone, no more error, display OK high resolution
vmalloc=256M + intel_iommu=off, no difference with vmalloc=256M alone
@Juanito : to my (limited) understanding, firmware execution is part of the boot process, so the required extensions shall be loaded 'on boot', no?
Anyway, everything seems fine now.
Thanks again for your prompt return.
EDIT : @Juanito : actually you are right, this works fine without loading the firmware-i915.tcz on boot. Mystery for me, this means that Linux already holds the firmware in the kernel?