drm/amd/display: Fix NULL ptr when calculating refresh rate

Calculate preferred refresh rate only when preferred mode exists.

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>