vt: use TERMINAL_DECLARE_EARLY() macro

It simplifies the declaration of the driver structures a little. There
are no current consumers of this macro, in fact it looks like it was
added for exactly this purpose.

This decreases the scope of some variables, so rework the initialization
in vt_init_logos() such that it doesn't require them.

No functional change intended.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34820

(cherry picked from commit ff1c8af8c34965cc13e53f8290adc5a2fe09501b)