lc-dec: Tune sf adaptive_luma_loop_filter_skip

In this patch, the speed feature 'adaptive_luma_loop_filter_skip'
is tuned by setting 'filter_threshold'=16 for 'pyramid_level'=5 and 6.
This change improves the visual quality for low complexity decode
mode.

Performance results of encoder and decoder for the streams generated
with low complexity decode mode enabled.

                 Decode Time              BD-Rate Loss(%)
cpu  Resolution  Reduction(%)  avg.psnr    ssim      vmaf    vmaf_neg
 1       608p      -1.111      -0.0494    -0.0716   0.0429    0.0096
 2       608p      -1.074      -0.0585    -0.0688   0.0113    0.0227
 3       608p      -1.700      -0.0423    -0.0580  -0.0438   -0.0179
 1       720p      -1.280      -0.0602    -0.0711   0.0296    0.0087
 2       720p      -1.343      -0.0559    -0.0660   0.0081    0.0066
 3       720p      -1.279      -0.0479    -0.0631   0.0041   -0.0067

STATS_CHANGED for vertical 608p and 720p with low complexity decode
mode enabled.

Change-Id: I8e426efb8ce011241549a505d11a05e750e00820

av1/encoder/picklpf.c

diff --git a/av1/encoder/picklpf.c b/av1/encoder/picklpf.c
index bc776d7..613613d 100644
--- a/av1/encoder/picklpf.c
+++ b/av1/encoder/picklpf.c
@@ -409,9 +409,7 @@
       unsigned int pyramid_level = cm->current_frame.pyramid_level;
       if (pyramid_level > 1) {
         int filter_threshold;
-        if (pyramid_level >= 5)
-          filter_threshold = 32;
-        else if (pyramid_level >= 4)
+        if (pyramid_level >= 4)
           filter_threshold = 16;
         else
           filter_threshold = 8;