Fences are required to support being released from under an atomic context.
The drm_atomic_state struct may take a mutex when being released and so
we cannot drop a reference to the drm_atomic_state from the fence release
path directly, and so we need to defer that unreference to a worker.

[  326.576697] WARNING: CPU: 2 PID: 366 at kernel/sched/core.c:7737 __might_sleep+0x5d/0x80
[  326.576816] do not call blocking ops when !TASK_RUNNING; state=1 set at [<ffffffffc0359549>] intel_breadcrumbs_signaler+0x59/0x270 [i915]
[  326.576818] Modules linked in: rfcomm fuse snd_hda_codec_hdmi bnep snd_hda_codec_realtek snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer input_leds led_class snd punit_atom_debug btusb btrtl btbcm btintel intel_rapl bluetooth i915 drm_kms_helper syscopyarea sysfillrect iwlwifi sysimgblt soundcore fb_sys_fops mei_txe cfg80211 drm pwm_lpss_platform pwm_lpss pinctrl_cherryview fjes acpi_pad parport_pc ppdev parport autofs4
[  326.576899] CPU: 2 PID: 366 Comm: i915/signal:0 Tainted: G     U          4.10.0-rc3-patser+ #5030
[  326.576902] Hardware name:                  /NUC5PPYB, BIOS PYBSWCEL.86A.0031.2015.0601.1712 06/01/2015
[  326.576905] Call Trace:
[  326.576920]  dump_stack+0x4d/0x6d
[  326.576926]  __warn+0xc0/0xe0
[  326.576931]  warn_slowpath_fmt+0x5a/0x80
[  326.577004]  ? intel_breadcrumbs_signaler+0x59/0x270 [i915]
[  326.577075]  ? intel_breadcrumbs_signaler+0x59/0x270 [i915]
[  326.577079]  __might_sleep+0x5d/0x80
[  326.577087]  mutex_lock+0x1b/0x40
[  326.577133]  drm_property_free_blob+0x1e/0x80 [drm]
[  326.577167]  ? drm_property_destroy+0xe0/0xe0 [drm]
[  326.577200]  drm_mode_object_unreference+0x5c/0x70 [drm]
[  326.577233]  drm_property_unreference_blob+0xe/0x10 [drm]
[  326.577260]  __drm_atomic_helper_crtc_destroy_state+0x14/0x40 [drm_kms_helper]
[  326.577278]  drm_atomic_helper_crtc_destroy_state+0x10/0x20 [drm_kms_helper]
[  326.577352]  intel_crtc_destroy_state+0x9/0x10 [i915]
[  326.577388]  drm_atomic_state_default_clear+0xea/0x1d0 [drm]
[  326.577462]  intel_atomic_state_clear+0xd/0x20 [i915]
[  326.577497]  drm_atomic_state_clear+0x1a/0x30 [drm]
[  326.577532]  __drm_atomic_state_free+0x13/0x60 [drm]
[  326.577607]  intel_atomic_commit_ready+0x6f/0x78 [i915]
[  326.577670]  i915_sw_fence_release+0x3a/0x50 [i915]
[  326.577733]  dma_i915_sw_fence_wake+0x39/0x80 [i915]
[  326.577741]  dma_fence_signal+0xda/0x120
[  326.577812]  ? intel_breadcrumbs_signaler+0x59/0x270 [i915]
[  326.577884]  intel_breadcrumbs_signaler+0xb1/0x270 [i915]
[  326.577889]  kthread+0x127/0x130
[  326.577961]  ? intel_engine_remove_wait+0x1a0/0x1a0 [i915]
[  326.577964]  ? kthread_stop+0x120/0x120
[  326.577970]  ret_from_fork+0x22/0x30

Fixes: c004a90b ("drm/i915: Restore nonblocking awaits for modesetting")
Reported-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170123212939.30345-1-chris@chris-wilson.co.uk
Cc: <drm-intel-fixes@lists.freedesktop.org> # v4.10-rc1+
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>