drm/vc4: crtc: Create vblank reporting function

We'll need that code in the HVS driver, so let's create a shared function
to reuse it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220711173939.1132294-15-maxime@cerno.tech