drm/atomic-helper: relax unregistered connector check

drm/atomic-helper: relax unregistered connector check

The driver might pull connectors which weren't submitted by
user-space into the atomic state. For instance,
intel_dp_mst_atomic_master_trans_check() pulls in connectors
sharing the same DP-MST stream. However, if the connector is
unregistered, this later fails with:

    [  559.425658] i915 0000:00:02.0: [drm:drm_atomic_helper_check_modeset] [CONNECTOR:378:DP-7] is not registered

Skip the unregistered connector check to allow user-space to turn
off connectors one-by-one.

See this wlroots issue:
https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3407

Previous discussion:
https://lore.kernel.org/intel-gfx/Y6GX7z17WmDSKwta@ideak-desk.fi.intel.com/

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231005131623.114379-1-contact@emersion.fr