drm/amdgpu: fix SPDX headers on amdgpu_cper.c/h

These should be MIT. The driver in general is MIT and
the license text at the top of the files is MIT so fix
it.

Fixes: 92d5d2a09de1 ("drm/amdgpu: Introduce funcs for populating CPER")
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4654
Reviewed-by: Mario Limonciello <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
(cherry picked from commit abd3f876404cafb107cb34bacb74706bfee11cbe)

drivers/gpu/drm/amd/amdgpu/amdgpu_cper.c

···
-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: MIT
 /*
  * Copyright 2025 Advanced Micro Devices, Inc.
  *

drivers/gpu/drm/amd/amdgpu/amdgpu_cper.h

···
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: MIT */
 /*
  * Copyright 2025 Advanced Micro Devices, Inc.
  *