drm/amd/display: Update 128b/132b downspread factor to 0.3%

[Why]
Updating downspread factor to 0.3% to add additional margin to account
for potential link rate deviations (up to 300ppm as per the DP spec).

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>