drm/vc4: crtc: Use an union to store the page flip callback

We'll need to extend the vc4_async_flip_state structure to rely on
another callback implementation, so let's move the current one into a
union.

Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220610115149.964394-10-maxime@cerno.tech