Sounds like this is the magic bit:
SubSection "extmod" Option "omit xorg-dga" # don't initialise the DGA extension EndSubSection
Could you please test this clean xorg.conf, which is a current stock xorg.conf just with the above bit added? If it makes it work, then we can forward this upstream for more advice. (Maybe it needs a quirk)
Sounds like this is the magic bit:
SubSection "extmod"
Option "omit xorg-dga" # don't initialise the DGA extension
EndSubSection
Could you please test this clean xorg.conf, which is a current stock xorg.conf just with the above bit added? If it makes it work, then we can forward this upstream for more advice. (Maybe it needs a quirk)