drm/i915/dp_mst: Fix connector initialization in intel_dp_add_mst_connector()

drm/i915/dp_mst: Fix connector initialization in intel_dp_add_mst_connector()

The connector initialization in intel_dp_add_mst_connector() depends on
the device pointer in connector to be valid, at least by connector
debug printing. The device pointer is initialized by drm_connector_init(),
however that function also exposes the connector to in-kernel users,
which can't be done before the connector is fully initialized. For now
make sure the device pointer is valid before it's used, until a
follow-up change moving this to DRM core.

This issue was revealed by the commit in the Fixes: line below, before
which the above debug printing checked and handled a NULL device pointer
gracefully in DRM core.

Cc: Jani Nikula <jani.nikula@intel.com>
Fixes: 529798bd786a ("drm/i915/mst: convert to struct intel_display")
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12799
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241118131044.1278028-1-imre.deak@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>