Fix a panic during boot caused by inadequate locking of some vt(4) driver
data structures.

vt_change_font() calls vtbuf_grow() to change some vt driver data
structures. It uses TF_MUTE to prevent the console from trying to use those
data structures while it changes them.

During the early stage of the boot process, the vt driver's tc_done routine
uses those data structures; however, it is currently called outside the
TF_MUTE check.

Move the tc_done routine inside the locked TF_MUTE check.

PR: 217282
Reviewed by: ed, ray
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D9709