interface_scrollable: Optimize calculations and remove redundant

interface_scrollable: Optimize calculations and remove redundant

Summary:
There are some redundant code blocks, unnessacery recursive logics
for interface_scrollable.

This commit removes redundant code blocks and serializes unnessacery
recursive logics

Test Plan:
 1. Run efl application
 2. Check that app works properly.
 2. Check how many pan_changed_cb calls are reduced.

Reviewers: cedric, jpeg, woohyun, herdsman

Subscribers: jsuya, eagleeye, z-wony

Differential Revision: https://phab.enlightenment.org/D4671

Change-Id: I70137b3ac17317404298d5abf0c9f29c742a6769
Signed-off-by: Wonki Kim <wonki_.kim@samsung.com>