When we have an OBMC block at the top of a tile, we don't generate
an 'above' prediction, in order to preserve tile independence.
However, the conditions for when to *generate* the above prediction
and when to *use* it were different when dependent-horztiles is
enabled.

Since dependent-horztiles allows each tile to depend on the tile
above, it seems reasonable to unify the conditions to use
xd->above_available (and similar for left preds, for the sake
of consistency). This allows OBMC across horizontal tile boundaries,
but not across tile group boundaries, in line with the rest of the
dependent-horztiles experiment.

Change-Id: I86d5de132c4429106a6025e7a7a2baec974e618c