drm/i915: Drain the freed state from the tail of the next commit

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.uk
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>