drm/nouveau/kms: Fix backlight registration

The nouveau code used to call drm_fb_helper_initial_config() from
nouveau_fbcon_init() before calling drm_dev_register(). This would
probe all connectors so that drm_connector->status could be used during
backlight registration which runs from nouveau_connector_late_register().

After commit 4a16dd9d ("drm/nouveau/kms: switch to drm fbdev helpers")
the fbdev emulation code, which now is a drm-client, can only run after
drm_dev_register(). So during backlight registration the connectors are
not probed yet and the drm_connector->status == connected check in
nv50_backlight_init() would now always fail.

Replace the drm_connector->status == connected check with
a drm_helper_probe_detect() == connected check to fix nv_backlight
no longer getting registered because of this.

Fixes: 4a16dd9d

 ("drm/nouveau/kms: switch to drm fbdev helpers")
Link: https://gitlab.freedesktop.org/drm/nouveau/-/issues/202
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2181941
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230326205433.36485-1-hdegoede@redhat.com