Before commit 2f5faff7f9 when the compositor is locked it would
reset the keyboard focus on all of the seats as part of pushing the
focus_state. This was removed because it now always keeps track of the
focus_state in the workspace instead of waiting until the state is
pushed. However this had the side effect that the active surface would
retain focus when the compositor is locked. This patch just makes it
explicitly set the keyboard focus to NULL on all of the seats when
locking. This will be restored based on the workspace's state when
unlocking.
https://bugs.freedesktop.org/show_bug.cgi?id=73905
b4a91708e3