vt(4): Call post-switch callback after replacing the backend

[Why]
For instance, it gives a chance to the new backend to refresh the
screen. This is needed by the vt_drmfb backend and drm_fb_helper.

This change was lost when I posted changes to reviews.freebsd.org and it
broken the amdgpu driver... Thanks to manu@ for reporting the problem
and wulf@ to find out the missing change!

Tested by: manu
Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D42834

(cherry picked from commit 40c20fc29cad4d38d9a565e9c7ba78612097308e)