Remove multiple coefficient buffers from PICK_MODE_CONTEXT

This commit is a manual cherry-pick from aom/master:
45592a39d3b00aee4d6bd70da669400017b7a5d8

Only part of the changes apply in nextgenv2

Change-Id: I1e22514c6fe5af556710254278f2f8a5805db999
diff --git a/av1/encoder/context_tree.h b/av1/encoder/context_tree.h
index 7453a66..7496d11 100644
--- a/av1/encoder/context_tree.h
+++ b/av1/encoder/context_tree.h
@@ -35,13 +35,13 @@
 #endif
 
   // dual buffer pointers, 0: in use, 1: best in store
-  tran_low_t *coeff[MAX_MB_PLANE][3];
-  tran_low_t *qcoeff[MAX_MB_PLANE][3];
-  tran_low_t *dqcoeff[MAX_MB_PLANE][3];
+  tran_low_t *coeff[MAX_MB_PLANE];
+  tran_low_t *qcoeff[MAX_MB_PLANE];
+  tran_low_t *dqcoeff[MAX_MB_PLANE];
 #if CONFIG_PVQ
   tran_low_t *pvq_ref_coeff[MAX_MB_PLANE];
 #endif
-  uint16_t *eobs[MAX_MB_PLANE][3];
+  uint16_t *eobs[MAX_MB_PLANE];
 
   int num_4x4_blk;
   int skip;