Fix touchscreen display on Pi3b on buster

[steveatinfincia]

Until now we have configured QOpenHD to run on the ground station using either the old broadcom drivers on stretch, or the new atomic DRM API on buster.

However for some reason the atomic api does not seem to work properly when running on the Pi3B with the pi touchscreen attached. The HDMI port works normally, as does Pi4B.

There does not appear to be any downside to using the non-atomic DRM API and just always setting the display mode using QT_QPA_EGLFS_ALWAYS_SET_MODE=1.