add definition to enforce both reflist not empty

it is a suppliment of #220
to distinguish P frame and low delay B frame

Signed-off-by: Carl Zhang <carl.zhang@intel.com>

va/va.h

@@ -1016,6 +1016,8 @@ typedef union _VAConfigAttribValEncJPEG {
 #define VA_PREDICTION_DIRECTION_PREVIOUS                0x00000001
 /** \brief Driver support backward prediction frame/slice */
 #define VA_PREDICTION_DIRECTION_FUTURE                  0x00000002
+/** \brief Dirver require both reference list must be not empty */
+#define VA_PREDICTION_DIRECTION_BI_NOT_EMPTY            0x00000004
 /**@}*/
 
 /** @name Attribute values for VAConfigAttribEncIntraRefresh */