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