drm: vc4: Free the dlist alloc immediately if it never hit the hw

atomic_check creates a state, and allocates the dlist memory for
it such that atomic_flush can not fail.

On destroy that dlist allocation was being put in the stale list,
even though it had never been programmed into the hardware,
therefore doing lots of atomic_checks could consume all the dlist
memory and fail.

If the dlist has never been programmed into the hardware, then
free it immediately.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>