vt: call driver's postswitch when panicking on ttyv0

In vt_kms, the postswitch callback restores fbdev mode when
panicking or entering the debugger. This ensures that even when
a graphical applicatino was running on the first tty, simple framebuffer
mode would be restored and the panic would be visible instead
of the frozen GUI. But vt wouldn't call the postswitch callback
when we're already on the first tty, so running a GUI on it
would prevent you from reading any panics.

Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D29961

(cherry picked from commit c937a405bdce2fd12c534446a8cc9e5548dd27eb)