drm/amd/display: Check for Zero Range in FreeSync Calc

-check for min/max range in freesync calculation and handle it accordingly

Signed-off-by: Eric <eric.cook@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>