drm/amdgpu: clarify license in amdgpu_trace_points.c

It was not clear.  The rest of the driver is MIT/X11.

Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>