drm/i915/mst: ack sink irq ESI for link status changes

Only specific event status indicators caused the link status to be
acked. Be sure to ack the link status change event.

Arguably we should track which bits to actually clear in ESI instead of
the wholesale approach.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220112110319.1172110-6-jani.nikula@intel.com