drm/amdgpu: apply umc_for_each_channel macro to umc_6_1

use umc_for_each_channel to make code simpler

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>