drm/amd/display: Program CW5 for tracebuffer for dcn20

[Why]
On dcn21 this is programmed for tracebuffer support but isn't being
programmed on dcn20.

DMCUB execution hits an undefined address 65000000 on tracebuffer
access.

[How]
Program CW5.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>