drm/i915: fix page flip finish vs. prepare on plane B

The refreshed patch had a copy & paste bug.

Reported-by: Simon Farnsworth <[e-mail]>
Signed-off-by: Jesse Barnes <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/gpu/drm/i915/i915_irq.c

···
 		}
 
 		if (iir & I915_DISPLAY_PLANE_B_FLIP_PENDING_INTERRUPT) {
+			intel_prepare_page_flip(dev, 1);
 			if (dev_priv->flip_pending_is_done)
 				intel_finish_page_flip_plane(dev, 1);
-			intel_prepare_page_flip(dev, 1);
 		}
 
 		if (pipea_stats & vblank_status) {