drm/xe: Use SPDX-License-Identifier instead of license text

Replace the license text with its SPDX-License-Identifier for
quick identification of the license and consistency with the
rest of the driver.

Reported-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>