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
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29961