drm/amd/display: used optimum VSTARTUP instead of MaxVStartup

[Description]
Features that are desirable for minimizing the Global Sync Period:
DRR and lateflip

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>