drm/vc4: Fix a framebuffer reference leak on async flip interrupt.

We'd need X to queue up an async pageflip while another is
outstanding, and then take a SIGIO.  I think X actually avoids sending
out the next pageflip while one's already queued, but I'm not sure.

Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 48627eb8dc)

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -527,6 +527,7 @@ static int vc4_async_page_flip(struct drm_crtc *crtc,
 	/* Make sure all other async modesetes have landed. */
 	ret = down_interruptible(&vc4->async_modeset);
 	if (ret) {
+		drm_framebuffer_unreference(fb);
 		kfree(flip_state);
 		return ret;
 	}