drm/i915/mst: only ack the ESI we actually handled

Seems odd that we clear all event status indicators if we've only
handled some. Only clear the ones we've handled.

v2: ack DOWN_REP and UP_REQ only if they were set in esi (Ville)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220120110102.3116218-1-jani.nikula@intel.com