drm/amd/display: treat memory as a single-channel for asymmetric memory

[Why]
1. Driver use umachannelnumber to calculate watermarks for stutter.
In asymmetric memory config, the actual bandwidth is less than
dual-channel. The bandwidth should be the same as single-channel.
2. We found single rank dimm need additional delay time for stutter.

[How]
Get information from each DIMM. Treat memory config as a single-channel
for asymmetric memory in bandwidth calculating. Add additional delay
time for single rank dimm.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Hugo Hu <hugo.hu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>