Revert "drm: hide unregistered connectors from GETCONNECTOR IOCTL"

This reverts commit 981f0929

.

It turns out this causes logically active but disconnected DP MST
connectors to disappear from the KMS resources list, and Mutter
then assumes the connector is already disabled. Later on Mutter tries
to re-use the same CRTC but fails since on the kernel side it's still
tied to the disconnected DP MST connector.

Signed-off-by: Simon Ser <contact@emersion.fr>
Tested-by: Jonas Ådahl <jadahl@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221017153150.60675-1-contact@emersion.fr