drm/connector: Add a way to init/add a connector in separate steps

Atm when the connector is added to the drm_mode_config::connector_list,
the connector may not be fully initialized yet. This is not a problem
for static connectors initialized/added during driver loading, for which
the driver ensures that look-ups via the above list are not possible
until all the connector and other required state is fully initialized
already. It's also not a problem for user space looking up either a
static or dynamic (see what this is below) connector, since this will be
only possible once the connector is registered.

A dynamic - atm only a DP MST - connector can be initialized and added
after the load time initialization is done. Such a connector may be
looked up by in-kernel users once it's added to the connector list. In
particular a hotplug handler could perform a detection on all the
connectors on the list and hence find a connector there which isn't yet
initialized. For instance the connector's helper hooks may be unset,
leading to a NULL dereference while the detect helper calls the
connector's drm_connector_helper_funcs::detect() or detect_ctx()
handler.

To resolve the above issue, add a way for dynamic connectors to
separately initialize the DRM core specific parts of the connector
without adding it to the connector list - by calling the new
drm_connector_dynamic_init() - and to add the connector to the list
later once all the initialization is complete and the connector is
registered - by calling the new drm_connector_dynamic_register().

Adding the above 2 functions was also motivated to make the distinction
of the interface between static and dynamic connectors clearer: Drivers
should manually initialize and register only dynamic connectors (with
the above 2 functions). A driver should only initialize a static
connector (with one of the drm_connector_init*, drmm_connector_init*
functions) while the registration of the connector will be done
automatically by DRM core.

v2: (Jani)
- Let initing DDC as well via drm_connector_init_core().
- Rename __drm_connector_init to drm_connector_init_core_and_add().

v3:
- Rename drm_connector_init_core() to drm_connector_dynamic_init().
  (Sima)
- Instead of exporting drm_connector_add(), move adding the connector
  to the registration step via a new drm_connector_dynamic_register().
  (Sima)
- Update drm_connector_dynamic_init()'s function documentation and the
  commit log according to the above changes.
- Update the commit log describing the problematic scenario during
  connector detection. (Maxime)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> (v1)
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241211230328.4012496-2-imre.deak@intel.com

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