drm/i915: Queue page flip work via a low latency, unbound workqueue

While user space has control over the scheduling priority of its page
flipping thread, the corresponding work the driver schedules for MMIO
flips always runs from the generic system workqueue which has some
scheduling overhead due it being CPU bound. This would hinder an
application that wants more stringent guarantees over flip timing (to
avoid missing a flip at the next frame count).

Fix this by scheduling the work from the unbound system workqueue
which provides for minimal scheduling latency.

v2:
- Use an unbound workqueue instead of a high-prio one. (Tvrtko, Chris)
v3:
- Use the system unbound wq instead of a dedicated one. (Maarten)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97775
Testcase: igt/kms_cursor_legacy
CC: Chris Wilson <chris@chris-wilson.co.uk>
CC: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
CC: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474372699-22841-1-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -12266,7 +12266,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 
 		work->flip_queued_req = i915_gem_active_get(&obj->last_write,
 							    &obj->base.dev->struct_mutex);
-		schedule_work(&work->mmio_work);
+		queue_work(system_unbound_wq, &work->mmio_work);
 	} else {
 		request = i915_gem_request_alloc(engine, engine->last_context);
 		if (IS_ERR(request)) {