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

@@ -227,7 +227,7 @@ typedef enum {
 #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

@@ -236,9 +236,11 @@ static INLINE int vp10_nmv_ctx(const uint8_t ref_mv_count,
   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;