Do nothing in vt_upgrade if there is no vt driver

Previously, if no drivers attached at boot we would panic with
"vtbuf_fill_locked begin.tp_row 0 must be < screen height 0".

PR: 192248
Reviewed by: ray
MFC after: 3 days
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D954