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: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>