drm/client: Test for connectors before sending hotplug event

Test for connectors in the client code and remove a similar test
from the generic fbdev emulation. Do nothing if the test fails.
Not having connectors indicates a driver bug.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125200415.14123-2-tzimmermann@suse.de