Fix bug in rd_pick_rect_partition()

Passed proper partition type to pick_sb_modes().

BUG=aomedia:2687

Change-Id: Id5bd1c556a677329ab48d8e41542c88c3e71efbd
diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index bd1b50d..7c63b6f 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -3062,7 +3062,7 @@
 
   // Obtain the best mode for the partition sub-block.
   pick_sb_modes(cpi, tile_data, x, mi_row, mi_col, &part_search_state->this_rdc,
-                PARTITION_HORZ, bsize, cur_partition_ctx, best_remain_rdcost,
+                partition_type, bsize, cur_partition_ctx, best_remain_rdcost,
                 PICK_MODE_RD);
   av1_rd_cost_update(x->rdmult, &part_search_state->this_rdc);