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.