vaapi_encode: Clean up the packed header configuration

Add a larger warning more clearly explaining the consequences of missing
packed header support in the driver.  Also only write the extradata if the
user actually requests it via the GLOBAL_HEADER flag.

libavcodec/vaapi_encode_mpeg2.c

@@ -615,8 +615,8 @@ static av_cold int vaapi_encode_mpeg2_init(AVCodecContext *avctx)
         return AVERROR(EINVAL);
     }
 
-    ctx->va_packed_headers = VA_ENC_PACKED_HEADER_SEQUENCE |
-                             VA_ENC_PACKED_HEADER_PICTURE;
+    ctx->desired_packed_headers = VA_ENC_PACKED_HEADER_SEQUENCE |
+                                  VA_ENC_PACKED_HEADER_PICTURE;
 
     ctx->surface_width  = FFALIGN(avctx->width,  16);
     ctx->surface_height = FFALIGN(avctx->height, 16);