drm/amd/display: remove dcn1 watermark sets b, c and d

Currently dcn1 will not switch between watermark sets so we can
save time by not calculating 3 extra sets.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@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>