ges-validate: Fix compiler warning caused by usage of wrong enum type

ges-validate: Fix compiler warning caused by usage of wrong enum type

ges-validate.c:237:22: error: implicit conversion from enumeration type
      'GESEdge' to different enumeration type 'GESEditMode'
      [-Werror,-Wenum-conversion]
  GESEditMode edge = GES_EDGE_NONE;
              ~~~~   ^~~~~~~~~~~~~
ges-validate.c:277:41: error: implicit conversion from enumeration type
      'GESEditMode' to different enumeration type 'GESEdge'
      [-Werror,-Wenum-conversion]
              new_layer_priority, mode, edge, position))) {
                                        ^~~~

https://bugzilla.gnome.org/show_bug.cgi?id=759758