I can confirm Tomasz Miąsko's observation. I changed the startup stanza in lightdm.conf to:
start on ((filesystem
and runlevel [!06]
and started dbus
and (drm-device-added card0 PRIMARY_DEVICE_FOR_DISPLAY=1
or stopped udev-fallback-graphics))
or runlevel PREVLEVEL=S)
I can confirm Tomasz Miąsko's observation. I changed the startup stanza in lightdm.conf to:
start on ((filesystem DEVICE_ FOR_DISPLAY= 1 graphics) )
and runlevel [!06]
and started dbus
and (drm-device-added card0 PRIMARY_
or stopped udev-fallback-
or runlevel PREVLEVEL=S)
and lightdm starts as expected.