Use aom_writer with pvq_encode_partition().

Change the pvq_encode_partition() function to take an aom_writer struct
 instead of an od_ec_enc struct.

Change-Id: I459d31c600467958c9a1cbebd632fec05e01f534
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 62486c5..305a9d7 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -2130,7 +2130,7 @@
               if (i == 0 || (!pvq->skip_rest &&
                              !(pvq->skip_dir & (1 << ((i - 1) % 3))))) {
                 pvq_encode_partition(
-                    &w->ec, pvq->qg[i], pvq->theta[i], pvq->max_theta[i],
+                    w, pvq->qg[i], pvq->theta[i], pvq->max_theta[i],
                     pvq->y + pvq->off[i], pvq->size[i], pvq->k[i], model, adapt,
                     exg + i, ext + i, robust || is_keyframe,
                     (plane != 0) * OD_TXSIZES * PVQ_MAX_PARTITIONS +