drm/etnaviv: free events the usual way in recover worker

Clearing the whole bitmap at once is only a minor optimization in
a path that should be extremely cold. Free the events by calling
event_free() instead of directly manipulating the completion count
and event bitmap.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>