drm/vc4: crtc: Move the BO Handling out of Common Page-Flip Handler

The function vc4_async_page_flip() handles asynchronous page-flips in
the vc4 driver.

However, it mixes some generic code with code that should only be run on
older generations that have the GPU handled by the vc4 driver.

Let's split the generic part out of vc4_async_page_flip() and into a
common function that we be reusable by an handler made for the BCM2711.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>