drm: use workqueue instead of slow-work

Workqueue can now handle high concurrency.  Convert drm_crtc_helper to
use system_nrt_wq instead of slow-work.  The conversion is mostly
straight forward.  One difference is that drm_helper_hpd_irq_event()
no longer blocks and can be called from any context.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 93a1a31..c707270 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -31,7 +31,6 @@
 #include <linux/idr.h>
 
 #include <linux/fb.h>
-#include <linux/slow-work.h>
 
 struct drm_device;
 struct drm_mode_set;
@@ -595,7 +594,7 @@
 
 	/* output poll support */
 	bool poll_enabled;
-	struct delayed_slow_work output_poll_slow_work;
+	struct delayed_work output_poll_work;
 
 	/* pointers to standard properties */
 	struct list_head property_blob_list;