panel-dt: fix number of found panel modes

The following errors can be observed in dmesg on some boards (e.g.
SANTINO/SANTINO-LT):
[    3.102655] /disp0: could not find display-timings node
[    3.108037] /disp0: no timings specified

It looks like panel_dt_get_non_edid_modes() was made from
panel_simple_get_non_edid_modes() of drivers/gpu/drm/panel/panel-simple.c
but increasing number of found modes was omited.

panel-simple.c has this code:
283-            if (mode) {
284-                    drm_mode_probed_add(connector, mode);
285:                    num = 1;
286-            } else {

So add the same assignment in panel-dt.c too.