drm/vc4: gem: Add a managed action to cleanup the job queue

The job queue needs to be cleaned up using vc4_gem_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_gem_destroy.

Fixes: d5b1a78a ("drm/vc4: Add support for drawing 3D frames.")
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-3-maxime@cerno.tech