This patch prepares for removing of av1_update_mv_context(). In
av1_update_mv_context() and av1_find_mv_refs(), the neighbouring
positions searched are not exactly the same. This patch fixes it.
This causes bitstream chamges, but shouldn't affect the coding
quality.

Change-Id: I59d2f8c318df388f2d06634cd96802b773c8bb13