Separate setup_scale_factor/extend_frame_borders

This commit takes out vp9_extend_frame_borders from
vp9_setup_scale_factors.

The refactoring is for the preparation of the use of lazy border
extension at decoder. This makes it necessary to handle border
extension separately at encoder/decoder. The use of
vp9_extend_frame_borders will be removed, when lazy border extension
is ready.

Change-Id: Ia3baba3d179d5f11eee1634f19b3b319d2a59186