drm/vc4: hvs: Move the dlist setup to its own function

The vc4_hvs_update_dlist function mostly deals with setting up the
vblank events and setting up the dlist entry pointer to our current
active one.

We'll want to do the former separately from the vblank handling in later
patches, so let's move it to a function of its own.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20220331143744.777652-6-maxime@cerno.tech