intel/eu: Plumb header present bit to codegen helpers for HDC messages.

intel/eu: Plumb header present bit to codegen helpers for HDC messages.

This makes sure that the header-present bit of the message descriptor
is in sync with the IR instruction fields, which gives the optimizer
more control to avoid the overhead of setting up a message header when
it's possible to do so.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>