drm/amd/display: correct DML calc error

[Why]
DML calculation is different from HW formula.

[How]
Correct the bug to keep it same as HW formula.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Sherry Wang <Yao.Wang1@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>