Use offset mode info when filling pc tree.

Use offset mode info when filling pc tree.

Use the appropriate subblock offset mode info rather than the parent
block base, when filling mbmi in the pc tree in nonrd_use_partition.
This mimics what is done in the vertical case and what is done for
both cases in nonrd_pick_partition.

This change has little practical effect at the moment since in speed 5
rt horizontal and vertical partitions are currently only used unpaired
at edges of the picture.

Change-Id: I4632f66ca84086dac56c7d36b45ddbe38a06f42a