If we have any residual freed atomic state from earlier commits, flush
the freed list after performing the current modeset. This prevents the
freed list from ever-growing if userspace manages to starve the kernel
threads (i.e. we are never able to run our free state worker and
eventually the system may even oom).

Fixes: eb955eee ("drm/i915: Move atomic state free from out of fence release")
Testcase: igt/kms_cursor/legacy/all-pipes-single-bo
Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170202204741.18231-1-chris@chris-wilson.co.ukReviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>