drm/amd/display: Update DML formula

[why]
This is to update SW DML implementation.

Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Ariel Bernstein <Eric.Bernstein@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>