Before an increased border of 288 pixels was used when both
CONFIG_EXT_PARTITION and CONFIG_FRAME_SUPERRES were enabled.

However increased border is also required when only
CONFIG_EXT_PARTITION is enabled.

For example when:
1) current frame is 2x smaller than reference frames
2) block size is 128x128

BUG=aomedia:991

Change-Id: I09dfddfdf6bd0b0efde2556acb924bb563b6da2f