Fixes the following EU validation error:
ERROR: Header must be present for all URB messages.
The message header is ignored for URB fence messages, so I doubt that
this actually matters in practice. But we should probably mark it as
present, because you have to send something, and according to the
documentation, there is a message header, it's just ignored.
Fixes: e6a9501aa2 ("intel/fs: Add the URB fence message")
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17624>