vt(4): Fix keyboard allocation when kbdmux(4) isn't used

The problem was that only the kbdmux keyboard index was saved in
vd->vd_keyboard. This index is -1 when kbdmux isn't used. In this
case, the keyboard was correctly allocated, but the returned index was
discarded.

PR:		194718
MFC of:		r273973
drm: Lower priority of three messages related to invalid EDID

Like in r259717, the prority goes from "error" to "debug" to avoid
spamming logs when the connectors are polled.

PR:		194770
Submitted by:	Larry Rosenman <ler@lerctr.org>
MFC of:		r273962, r274587