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

sys/dev/vt/vt_core.c

@@ -3283,6 +3283,13 @@ vt_replace_backend(const struct vt_driver *drv, void *softc)
 	/* Update windows sizes and initialize last items. */
 	vt_upgrade(vd);
 
+	/*
+	 * Give a chance to the new backend to run the post-switch code, for
+	 * instance to refresh the screen.
+	 */
+	if (vd->vd_driver->vd_postswitch)
+		vd->vd_driver->vd_postswitch(vd);
+
 #ifdef DEV_SPLASH
 	if (vd->vd_flags & VDF_SPLASH)
 		vtterm_splash(vd);