palette-delta-encoding: remove dependency on above SB row

Do not refer to above MB when on SB boundary. This helps to
reduce line buffer.

Keyframe coding gain on the screen_content testset drops from
1.28% to 1.20%.

Change-Id: I4b06fd137d6c9ea68d618035381f09d0746ba9e8

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 9017eed..3e982e3 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -1394,10 +1394,8 @@
                                    const PALETTE_MODE_INFO *const pmi,
                                    int bit_depth, aom_writer *w) {
   const int n = pmi->palette_size[0];
-  const MODE_INFO *const above_mi = xd->above_mi;
-  const MODE_INFO *const left_mi = xd->left_mi;
   uint16_t color_cache[2 * PALETTE_MAX_SIZE];
-  const int n_cache = av1_get_palette_cache(above_mi, left_mi, 0, color_cache);
+  const int n_cache = av1_get_palette_cache(xd, 0, color_cache);
   int out_cache_colors[PALETTE_MAX_SIZE];
   uint8_t cache_color_found[2 * PALETTE_MAX_SIZE];
   const int n_out_cache =
@@ -1423,10 +1421,8 @@
   const uint16_t *colors_u = pmi->palette_colors + PALETTE_MAX_SIZE;
   const uint16_t *colors_v = pmi->palette_colors + 2 * PALETTE_MAX_SIZE;
   // U channel colors.
-  const MODE_INFO *const above_mi = xd->above_mi;
-  const MODE_INFO *const left_mi = xd->left_mi;
   uint16_t color_cache[2 * PALETTE_MAX_SIZE];
-  const int n_cache = av1_get_palette_cache(above_mi, left_mi, 1, color_cache);
+  const int n_cache = av1_get_palette_cache(xd, 1, color_cache);
   int out_cache_colors[PALETTE_MAX_SIZE];
   uint8_t cache_color_found[2 * PALETTE_MAX_SIZE];
   const int n_out_cache = av1_index_color_cache(