For sub-8x8 blocks, scan loop should have at least one iteration.
Previously, loop would be skipped if col_offset == 1 and n8_w == 1.

This change appears to have neutral effect on bdrate (+0.01).  This could
mean that extra MV candidates are not helping that much for sub-8x8.

Change-Id: Ic778312b39113ab8f348f5c7c18813593a831b7c