video: Factor out vidconsole_put_string()

Pull the vidconsole_put_string() function from DM tests, make it
available to e.g. boards that want to display information on the
LCD on boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Anatolij Gustschin <agust@denx.de>