vt(4): Call vd_postswitch callback regardless of the current window

[Why]
We want the same behavior at the backend level, regardless if we need to
switch the current window or not.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D42751

(cherry picked from commit 16b13bd3fd63be56653296ba54956f5526918445)