drm/i915: Fix misplaced intel_mark_page_flip_active()

In commit 28c4566d, backport of commit e7d841ca ('drm/i915:
Close race between processing unpin task and queueing the flip') I
somehow added two calls to intel_mark_page_flip_active() from
intel_gen4_queue_flip() and none from intel_gen6_queue_flip().  There
should of course be one from each.
Reported-by: Julien Cristau <jcristau@debian.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -7155,8 +7155,6 @@ static int intel_gen4_queue_flip(struct drm_device *dev,
 	pipesrc = I915_READ(PIPESRC(intel_crtc->pipe)) & 0x0fff0fff;
 	OUT_RING(pf | pipesrc);
 
-	intel_mark_page_flip_active(intel_crtc);
-
 	intel_mark_page_flip_active(intel_crtc);
 	ADVANCE_LP_RING();
 	return 0;
@@ -7193,6 +7191,8 @@ static int intel_gen6_queue_flip(struct drm_device *dev,
 	pf = I915_READ(PF_CTL(intel_crtc->pipe)) & PF_ENABLE;
 	pipesrc = I915_READ(PIPESRC(intel_crtc->pipe)) & 0x0fff0fff;
 	OUT_RING(pf | pipesrc);
+
+	intel_mark_page_flip_active(intel_crtc);
 	ADVANCE_LP_RING();
 	return 0;