drm: Really never disable vblank irqs for offdelay==0

With the new support for immediate vblank disabling we always disabled
the vblank interrupt right away, irrespective of the vblank offdelay
setting.

But being able to let vblanks run forever is fairly useful for
debugging, so restore that behaviour.
Suggested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_irq.c

@@ -1005,9 +1005,11 @@ void drm_vblank_put(struct drm_device *dev, int crtc)
 
 	/* Last user schedules interrupt disable */
 	if (atomic_dec_and_test(&vblank->refcount)) {
-		if (dev->vblank_disable_immediate || drm_vblank_offdelay < 0)
+		if (drm_vblank_offdelay == 0)
+			return;
+		else if (dev->vblank_disable_immediate || drm_vblank_offdelay < 0)
 			vblank_disable_fn((unsigned long)vblank);
-		else if (drm_vblank_offdelay > 0)
+		else
 			mod_timer(&vblank->disable_timer,
 				  jiffies + ((drm_vblank_offdelay * HZ)/1000));
 	}