drm/panel: simple: Check against num_timings when setting preferred for timing

In the loop on .timings, we should check .num_timings to see if it's the
only mode specified, not .num_modes, which should be used with .modes.

Fixes: cda55372 ("drm/panel: simple: Set appropriate mode type")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>