graph-depends: rename the mode constants

This is ugly, since Python does not have enum constructs, so by moving
the 'type' of the constant ('MODE' here) to the beginning, we get an
artificial 'namespace' for the constants.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>