drm/amd/display: single channel bandwidth verses dual channel bandwidth

DPM0, FCLK=MCLK, single channel bandwidth = dual channel bandwidth
for the rest of the DPM levels, single channel bandwidth = 1/2  dual channel bandwidth

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>