tty: Move the vt_tty field from the vc_data into the standard tty_port

This takes all the tty references through the expected interface points so
we can refcount them.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>