vt(4): Always call vt_window_switch() in vtterm_cnungrab()

[Why]
This ensures that vtterm_cnungrab() is the mirror of vtterm_cngrab().
And the latter always call vt_window_switch() and thus the backend's
vd_postswitch().

This makes sure that whatever the backend did during vtterm_cngrab(), it
can undo it during vtterm_cnungrab().

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D42752