xrandr: Prevent guint32 overflow

Passing "9223372036854775807" (G_MAXINT64) as the timestamp will
overflow as the timestamp is cast to a guint32 internally. Clamp
that value instead to avoid g-s-d crashing.

See https://bugzilla.redhat.com/show_bug.cgi?id=1022562

https://bugzilla.gnome.org/show_bug.cgi?id=711065