drm/amdgpu: loose check for umc poison mode

No need to check poison setting for each channel, check for umc0
channel0 is enough.

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>