drm/amdgpu: fix license on Kconfig and Makefiles

amdgpu is MIT licensed.

Fixes: ec8f24b7

 ("treewide: Add SPDX license identifier - Makefile/Kconfig")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>