drm/amdgpu: cleanup the sync code

No need for two functions doing the same, remove one and
add comments what those functions actually do.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>