drm/xe: remove header variable from parse_g2h_msg

The header variable is unused, remove it.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Suggested-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>