drm/amd/display: Use the correct max downscaling value for DCN3.x family

[why]
As per spec, DCN3.x can do 6:1 downscaling and DCN2.x can do 4:1. The
max downscaling limit value for DCN2.x is 250, which means it's
calculated as 1000 / 4 = 250. For DCN3.x this then gives 1000 / 6 = 167.

[how]
Set maximum downscaling limit to 167 for DCN3.x

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org