gpu: ion: dereferencing an ERR_PTR

We dereference "heap->task" before checking if it's an ERR_PTR.

Fixes: ea313b5f ('gpu: ion: Also shrink memory cached in the deferred free list')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>