drm: vblank cannot be enabled if dev->irq_enabled is false

Since we cannot enable the vblank if !dev->irq_enabled, we assert that
checking for both !vblank->enabled and !dev->irq_enabled is tautological
and only need the former. The only time it may differ is when racing
with drm_irq_uninstall(), but that will then disable the vblank and
wakeup the waiters.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317202030.24410-2-chris@chris-wilson.co.ukReviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/drm_irq.c

@@ -1639,10 +1639,9 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 		DRM_DEBUG("waiting on vblank count %u, crtc %u\n",
 			  vblwait->request.sequence, pipe);
 		DRM_WAIT_ON(ret, vblank->queue, 3 * HZ,
-			    (((drm_vblank_count(dev, pipe) -
-			       vblwait->request.sequence) <= (1 << 23)) ||
-			     !READ_ONCE(vblank->enabled) ||
-			     !dev->irq_enabled));
+			    (drm_vblank_count(dev, pipe) -
+			     vblwait->request.sequence) <= (1 << 23) ||
+			    !READ_ONCE(vblank->enabled));
 	}
 
 	if (ret != -EINTR) {