gpu: ion: fix kfree/list_del order

With CONFIG_SLUB_DEBUG_ON it would panic during
ion_alloc()
 ion_buffer_create()
   io_heap_drain_freelist()

Signed-off-by: JP Abgrall <jpa@google.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>