Use 0 initial levels for loop filter search in keyframes

This fixes a discrepancy between GOP serial and parallel encoding.
(GOP serial encoding is changed to match parallel encoding.

BUG=aomedia:3004

STATS_CHANGED

Change-Id: I8475cc3ddab0f03862f52da062b1e6be1a5408c6

av1/encoder/picklpf.c

diff --git a/av1/encoder/picklpf.c b/av1/encoder/picklpf.c
index 4391ae9..30ce10f 100644
--- a/av1/encoder/picklpf.c
+++ b/av1/encoder/picklpf.c
@@ -272,10 +272,13 @@
     lf->filter_level_u = clamp(filt_guess, min_filter_level, max_filter_level);
     lf->filter_level_v = clamp(filt_guess, min_filter_level, max_filter_level);
   } else {
-    const int last_frame_filter_level[4] = { lf->filter_level[0],
-                                             lf->filter_level[1],
-                                             lf->filter_level_u,
-                                             lf->filter_level_v };
+    int last_frame_filter_level[4] = { 0 };
+    if (!frame_is_intra_only(cm)) {
+      last_frame_filter_level[0] = lf->filter_level[0];
+      last_frame_filter_level[1] = lf->filter_level[1];
+      last_frame_filter_level[2] = lf->filter_level_u;
+      last_frame_filter_level[3] = lf->filter_level_v;
+    }
 
     lf->filter_level[0] = lf->filter_level[1] =
         search_filter_level(sd, cpi, method == LPF_PICK_FROM_SUBIMAGE,