eglfs_kms should pick the correct card0/card1 automatically when only one can render

On the Raspberry Pi 4 there are two nodes under /dev/dri, card0 and card1. One can render, the other cannot. The order is random, sometimes Qt applications should be run with card0, sometimes with card1, and it may again become different after rebooting.

This means one has to constantly keep the config file (set in QT_QPA_EGLFS_KMS_CONFIG) up-to-date so that it contains the correct device. Ideally we should have some logic that, if no explicit device is specified in the config (or no config file is provided), we try to pick the right thing, not just /dev/dri/card0 always.

https://baek9.github.io/en/2018/01/finding-the-drm-device-node-corresponding-to-the-graphics-card/

Problem is, this involves udev and we cannot do that on the eglfs backend's level directly since it is optional. (input uses it, but it's optional and there's a non-udev path)