drm/i915: Wait for vblank before unpinning old fb

Be paranoid and ensure that the vblank has passed and the scanout has
switched to the new fb, before unpinning the old one and possibly
tearing down its PTEs.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -1626,8 +1626,10 @@ intel_pipe_set_base(struct drm_crtc *crtc, int x, int y,
 		return ret;
 	}
 
-	if (old_fb)
+	if (old_fb) {
+		intel_wait_for_vblank(dev, intel_crtc->pipe);
 		i915_gem_object_unpin(to_intel_framebuffer(old_fb)->obj);
+	}
 
 	mutex_unlock(&dev->struct_mutex);