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()

Change-Id: I3ff69652bd11137dc5bdde46eca6dd2d2de615be
Signed-off-by: JP Abgrall <jpa@google.com>