interface_scrollable: Unify basis of calculation of page_get logic

interface_scrollable: Unify basis of calculation of page_get logic

if rtl mode is set, current_page_get api will return page number by
substracting by result of last_page_get api to reverse x axis.

but if content size is not a integer multiple of page size,
(which means there is a partial size of the page)
current_page_get api returns a little bit large value.

this commit reverse x axis before calculation logic to unify a kind of
basis of axis.

@tizen_fix

Change-Id: I8a48d52829a8dc9d2660a1a77b44f42587459711
Signed-off-by: Wonki Kim <wonki_.kim@samsung.com>