vt(4): Skip vt_window_switch() for nested panics

[dumbbell]

Summary

Why

The same protection was added to vt_flush() in the previous commit (D42056). We want the same one in vt_window_switch(): if e.g. the DRM driver panics while handling a call to vt_window_switch(), we don't want to recursively call vt_window_switch() again and trigger another panic.