drm/i915/ddi: change intel_ddi_init_{dp, hdmi}_connector() return type

drm/i915/ddi: change intel_ddi_init_{dp, hdmi}_connector() return type

The caller doesn't actually need the returned struct intel_connector;
it's stored in the ->attached_connector of intel_dp and
intel_hdmi. Switch to returning an int with 0 for success and negative
errors codes to be able to indicate success even when we don't have a
connector.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/8ef7fe838231919e85eaead640c51ad3e4550d27.1735568047.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>