drm/vc4: crtc: Move the BO handling out of common page-flip callback

We'll soon introduce another completion callback source that won't need
to use the BO reference counting, so let's move it around to create a
function we will be able to share between both callbacks.

Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220610115149.964394-11-maxime@cerno.tech