Commit 70861874 ("gdm-wayland-session,gdm-x-session: register after
delay") delayed displays changing their status from PREPARED to MANAGED
so that their status would not change until the session has had a change
to install its own framebuffer and tell the GPU to scanout this new fb.

Commit 74ee7771 ("local-display-factory: defer killing greeter until
new session registers") uses this to avoid a flicker when transitioning
from the greeter to the user-session by deferring the stopping of the
greeter-session until the new display moves to the MANAGED state.

But this only works when transitioning to a new user-session, when moving
to an existing user-session (fast user switching) the display already
is in MANAGED state and instead of deferring the stopping of the greeter
commit 74ee7771 causes us to now never stop the greeter-session.

This commit fixes this by starting a timeout when switching away from
the initial-vt and letting that timeout stop the greeter-session.

This commit removes the finish_waiting_displays_on_seat() call when the
display's status changes to MANAGED, so that we still only have one code
path stopping the greeter and not two.

This means we also no longer need to delay registering the display. So this
commit removes the code adding the delay (reverts commit 74ee7771).

Note this commit uses a delay of 10 seconds, rather then 2 seconds. The
transition to a new user-session takes about 8 seconds on my budget
Apollo Lake based laptop (with SSD).

Note this all really is a workaround, the proper solution for this would
be able to tell the kernel to keep the greeter framebuffer around until
a new framebuffer is installed. There is a patch to add a new unref_fb
ioctl for this: https://www.spinics.net/lists/dri-devel/msg140912.html .
We need to get this patch upstream and teach mutter to use it.