amdgpu: update copyright date

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>

LICENSE.amdgpu

-Copyright (C) 2017  Advanced Micro Devices, Inc. All rights reserved.
+Copyright (C) 2018  Advanced Micro Devices, Inc. All rights reserved.
 
 REDISTRIBUTION: Permission is hereby granted, free of any license fees,
 to any person obtaining a copy of this microcode (the "Software"), to