Move number of memory channel calculation out of dcn_bw_update_from_pplib

Fill in fabric_and_dram_bandwidth for single channel case.
Signed-off-by: NEric Yang <Eric.Yang2@amd.com>
Reviewed-by: NTony Cheng <Tony.Cheng@amd.com>
Acked-by: NHarry Wentland <Harry.Wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>