power: don't assume we have a Screen proxy when IIO proxy is ready

On panel construction, we trigger asynchronous creation of a GDBusProxy
for the IIO sensors daemon and one for the Screen g-s-d interface.
Since there's no guarantee we'll have the screen proxy ready by the time
the IIO proxy is constructed, guard for (priv->screen_proxy != NULL), or
we'll hit a critical.

https://bugzilla.gnome.org/show_bug.cgi?id=754735