drm/amdgpu/dc: Revert commit "treat memory as a single-channel"

This reverts commit "dc: treat memory as a single-channel for
asymmetric memory".

Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>