hwcontext_vaapi: add the fourcc of I420 format map.

VA-API 2.0 have enable the I420, so enable this map.
Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>

libavutil/hwcontext_vaapi.c

@@ -101,7 +101,9 @@ static const struct {
     MAP(NV12, YUV420,  NV12),
     MAP(YV12, YUV420,  YUV420P), // With U/V planes swapped.
     MAP(IYUV, YUV420,  YUV420P),
-  //MAP(I420, YUV420,  YUV420P), // Not in libva but used by Intel driver.
+#ifdef VA_FOURCC_I420
+    MAP(I420, YUV420,  YUV420P),
+#endif
 #ifdef VA_FOURCC_YV16
     MAP(YV16, YUV422,  YUV422P), // With U/V planes swapped.
 #endif