drm/vc4: bo: Add a managed action to cleanup the cache

The BO cache needs to be cleaned up using vc4_bo_cache_destroy, but it's
not used consistently (vc4_drv's bind calls it in its error path, but
doesn't in unbind), and we can make that automatic through a managed
action. Let's remove the requirement to call vc4_bo_cache_destroy.

Fixes: c826a6e1

 ("drm/vc4: Add a BO cache.")
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-1-maxime@cerno.tech