i915/display/hotplug: use drm_kms_helper_connector_hotplug_event()

This adds more information to the hotplug uevent and lets user-space
know that it's about a particular connector only.

v2: don't rely on the changed HPD pin bitmask to count changed
    connectors (Jani)

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230623094229.120264-1-contact@emersion.fr