drm/amdgpu: add missing license to some files

The driver is MIT, so add the licenses.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/2265


Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>