tty: vt, do not pass structure over stack

The compiler noticed passing structure over stack. Even though rgb is
a small structure, let us define one and pass that over all the
functions wherever needed.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>