genlist: fix Issue "In Tree effect feature, items floates when number of item crosses...

genlist: fix Issue "In Tree effect feature, items floates when number of item crosses item block size".

Summary: "In Tree effect feature, during expansion of tree, when total number of items crosses block count, items start floating".

Test Plan: Run elementary_test, open Genlist Tree, keep expanding items untill it crosses number of count 32 (Block count). Resize the window, floting items can be seen.

Reviewers: raster, cedric

Subscribers: shilpasingh, poornima.srinivasan, govi, rajeshps

Differential Revision: https://phab.enlightenment.org/D2627

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>