Fix tile context pointer assignment in enc tile-mt

In tile based multi-threading of encoder, td->tctx is never used.
Removing the dependency on td->tctx and assigning the tile-context
pointer directly to xd->tile_ctx and x->tile_pb_ctx would facilitate
the switching between enc row-mt and tile-mt at frame level.

Change-Id: I7011e0dda83b4fea37700ab5a97e219a1c1a9757
diff --git a/av1/encoder/ethread.c b/av1/encoder/ethread.c
index 7b85f7e..b9cf178 100644
--- a/av1/encoder/ethread.c
+++ b/av1/encoder/ethread.c
@@ -356,9 +356,8 @@
 
     TileDataEnc *const this_tile =
         &cpi->tile_data[tile_row * cm->tile_cols + tile_col];
-    thread_data->td->tctx = &this_tile->tctx;
-    thread_data->td->mb.e_mbd.tile_ctx = thread_data->td->tctx;
-    thread_data->td->mb.tile_pb_ctx = thread_data->td->tctx;
+    thread_data->td->mb.e_mbd.tile_ctx = &this_tile->tctx;
+    thread_data->td->mb.tile_pb_ctx = &this_tile->tctx;
     thread_data->td->mb.backup_tile_ctx = &this_tile->backup_tctx;
     av1_encode_tile(cpi, thread_data->td, tile_row, tile_col);
   }