VP9: Pass NULL scale_factors ptr when not scaling

to vp9_setup_pre_planes(), preventing the function
unscaled_value() from being called.  unscaled_value()
returns the same value that was passed in.  See
scaled_buffer_offset() in vp9_reconinter.h.

Change-Id: I2a6fbaf07972c2f212834929d29a2cbe72e399c3

vp9/decoder/vp9_decodeframe.c

diff --git a/vp9/decoder/vp9_decodeframe.c b/vp9/decoder/vp9_decodeframe.c
index 39710c8..8a492d5 100644
--- a/vp9/decoder/vp9_decodeframe.c
+++ b/vp9/decoder/vp9_decodeframe.c
@@ -737,7 +737,8 @@
                          "Reference frame has invalid dimensions");
 
     is_scaled = vp9_is_scaled(sf);
-    vp9_setup_pre_planes(xd, ref, ref_buf->buf, mi_row, mi_col, sf);
+    vp9_setup_pre_planes(xd, ref, ref_buf->buf, mi_row, mi_col,
+                         is_scaled ? sf : NULL);
     xd->block_refs[ref] = ref_buf;
 
     if (sb_type < BLOCK_8X8) {