drm/amd/display: sometime VtotalMin less than VTotal (rounding issue)

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Jordan Lazare <Jordan.Lazare@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>