drm/xe: Update SPDX deprecated license identifier

The "GPL-2.0" SPDX license identifier is deprecated. Update the
code to use "GPL-2.0-only" instead. Choose this identifier over
"GPL-2.0-or-later" since it's the most restrictive of the two and it's
not fully clear that "GPL-2.0" also allows "GPL-2.0-or-later".

Cc: Francois Dugast <[e-mail]>
Cc: Rodrigo Vivi <[e-mail]>
Signed-off-by: Thomas Hellström <[e-mail]>
Reviewed-by: Francois Dugast <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/xe/xe_pt_walk.c

···
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright © 2022 Intel Corporation
  */

drivers/gpu/drm/xe/xe_pt_walk.h

···
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * Copyright © 2022 Intel Corporation
  */

drivers/gpu/drm/xe/xe_res_cursor.h

···
-/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+/* SPDX-License-Identifier: GPL-2.0-only OR MIT */
 /*
  * Copyright 2020 Advanced Micro Devices, Inc.
  *

drivers/gpu/drm/xe/xe_trace.c

···
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright © 2022 Intel Corporation
  */

drivers/gpu/drm/xe/xe_trace.h

···
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * Copyright © 2022 Intel Corporation
  */