evas_vg_node: Move change flag value setting

evas_vg_node: Move change flag value setting

Summary:
_node_change function is only executed
when the flag is none to prevent duplicate calls.
If CHANGE_MATRIX_FLAG is added to the flag in advance,
_node_change function does not operate normally.

Test Plan:
cd ./src/examples/evas
gcc -o evas_vg_simple evas-vg-simple.c `pkg-config --libs --cflags evas ecore ecore-evas eina ector eo efl` -lm
./evas_vg_simple

Reviewers: Hermet, kimcinoo, smohanty

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8895

Change-Id: I84a5c78026158522ae7347ed701b9662c6df4517