drm/i915: fix a possible refcount leak in intel_dp_add_mst_connector()

If drm_connector_init fails, intel_connector_free will be called to take
care of proper free. So it is necessary to drop the refcount of port
before intel_connector_free.

Fixes: 091a4f91

 ("drm/i915: Handle drm-layer errors in intel_dp_add_mst_connector")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220624130406.17996-1-jose.souza@intel.com
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
(cherry picked from commit cea9ed61

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>