drm/amdgpu: correct comment to clear up the confusion

Former comment looks to be one intended behavior in code,
actually it's not. So correct it.

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>