drm/vc4: hdmi: Switch to DRM-managed kfree to build regsets

The current code to build the registers set later exposed in debugfs for
the HDMI controller relies on traditional allocations, that are later
free'd as part of the driver unbind hook.

Since krealloc doesn't have a DRM-managed equivalent, let's add an action
to free the buffer later on.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>