Restrict # of neighbors in obmc blending

Only blend with the first N neighbors at each side. If the size of
one dimenstion is 8/16/32/64, the max # of neighbors to overlap
with is 1/2/3/4.
Previously we disable obmc mode if there are too many neighbors.

Change of performance in AWCY, compared to disabling obmc if
at any side there are more than 2 overlappable neighbors.
HL improved by 0.02%
LL improved by 0.09%

Change-Id: I93d9a65c6c4aabf0b4a4946e2253d3e2ef21a662