If plymouth doesn't get explicitly "activated" then when
GDM tries to deactivate it, the deactivation request is
a noop.

One aspect of being active, though is having ownership and
control of the terminal.  This happens immediately, even
before a splash is shown.

The `deactivate` request needs to relinguish such control,
unconditionally, since some display server is about to use
the tty.

This commit fixes that.