HW does not support < 4x4 prediction. To limit the bandwidth
requirement, for small blocks, only blend with neighbors from one
side. If block-size of current plane is 4x4 or 8x4, the above
neighbor will be skipped. If it is 4x8, the left neighbor (dir = 1)
will be skipped.
This change will keep the bandwidth requirement of OBMC not more
than what is required by normal compound inter modes.

Loss of gain (PSNR-Y/PSNR-Cb/PSNR-Cr/CIEDE2000)
AWCY HL: 0.05/0.09/0.14/0.04
AWCY LL: 0.06/0.07/0.10/0.12

Change-Id: I3854afc69c3014da99bde4b19bb726e4c077d59e