Fix panic on load new driver while vt(4) is in VGA textmode.
o Mute terminal while vt(4) driver change in progress.
o Reset VDF_TEXTMODE before init new driver.
o Assign default font, if new driver is not in TEXTMODE.
o Do not update screen while driver changing.

Resolved by:	adrian
Reported by:	tyler
MFC after:	7 days
Sponsored by:	The FreeBSD Foundation