Who are the OEM of Dell?
The answer would depend on the screen size.
Currently 15" OEM suppliers are Philips, Samsung and LiteOn. There will be a new one - BenQ later this year.
17" OEM suppliers are LiteOn and Samsung.
I have no idea who OEM 18" for Dell.
20" is BenQ, but I am not sure about this one.

Currently both CPT and HS panels were approved by Dell.
Since BenQ will be OEM for Dell soon, I assume AU passed the qualification.