Fix a bug in determination of reference_mode

Fix a bug where the encoder sets reference_mode =
COMPOUND_REFERENCE even if the frame contains some
4x4 blocks (but no larger blocks) that are single
reference. This occurs only in corner cases,
typically for very small frame sizes (e.g. 16x8).

BUG=aomedia:575

Change-Id: I5caedaa753a56e9f9b48995c1d2e0a0886aa4366