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_mjpeg.c

@@ -389,7 +389,7 @@ static av_cold int vaapi_encode_mjpeg_init(AVCodecContext *avctx)
     ctx->codec = &vaapi_encode_type_mjpeg;
 
     // The JPEG image header - see note above.
-    ctx->va_packed_headers =
+    ctx->desired_packed_headers =
         VA_ENC_PACKED_HEADER_RAW_DATA;
 
     ctx->surface_width  = FFALIGN(avctx->width,  8);