Deal correctly with 16x4/4x16 blocks in mvref_common.c

The logic that searches for motion vectors in scan_row_mbmi and
scan_col_mbmi steps in 4x4 units when the block size is less than
8x8. That test (bsize < BLOCK_8X8) doesn't work when you have 16X4 or
4X16 blocks, because they appear higher up in the enum.

Change-Id: Idb1a97f8a43b675fd78dbc76ae501fcdff7adbcd