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 48627eb8)