Enable context based motion vector entropy coding

This commit enables a context based motion vector entropy coding
conditioned on dynamic reference motion vector list. This (along with
the previous CL) imporves the coding gains due to dynamic motion
vector referencing based entropy coding:
derf   0.1%
hevcmr 0.2%
stdhd  0.7%
hevchr 0.4%

No encoding time change was observed.

Change-Id: I179c723844079195f6952a12582996a3ca9e9914

vp10/common/enums.h

diff --git a/vp10/common/enums.h b/vp10/common/enums.h
index e1f3168..6d7d08c 100644
--- a/vp10/common/enums.h
+++ b/vp10/common/enums.h
@@ -227,7 +227,7 @@
 #define SKIP_CONTEXTS 3
 
 #if CONFIG_REF_MV
-#define NMV_CONTEXTS 2
+#define NMV_CONTEXTS 3
 
 #define NEWMV_MODE_CONTEXTS  7
 #define ZEROMV_MODE_CONTEXTS 2

vp10/common/mvref_common.h

diff --git a/vp10/common/mvref_common.h b/vp10/common/mvref_common.h
index b3a8beb..bc6d824 100644
--- a/vp10/common/mvref_common.h
+++ b/vp10/common/mvref_common.h
@@ -236,9 +236,11 @@
   if (ref_mv_stack[0].weight > REF_CAT_LEVEL &&
       ref_mv_count > 0) {
     if (abs(ref_mv_stack[0].this_mv.as_mv.row -
-            ref_mv_stack[0].pred_mv.as_mv.row) < 8 &&
-        abs(ref_mv_stack[0].this_mv.as_mv.col -
-            ref_mv_stack[0].pred_mv.as_mv.col) < 8)
+             ref_mv_stack[0].pred_mv.as_mv.row) <= 4 &&
+         abs(ref_mv_stack[0].this_mv.as_mv.col -
+             ref_mv_stack[0].pred_mv.as_mv.col) <= 4)
+      return 2;
+    else
       return 1;
   }
   return 0;