Add configuration to config.txt to enable both HDMI outputs.
The KMS atomic API needs to be disabled as it fails with
dual HDMI output.

Taskk-number: QTBUG-82816
Change-Id: I1a9cc95eecd9fad253862da764f52bebb5cdd1df
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>