drm/amd/display: Formula refactor for calculating DPP CLK DTO

[Why]
Previous formula for calculating DPP CLK DTO was
hard to understand.

[How]
Replace with easier to understand formula that produces
same results.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>