dmatest: use pseudo random numbers

There is no need for dmatest to drain the entropy pool.

It would be nice to one day have repeatable runs, but would need a
larger rework to synchronize and order calls to the rng across test
threads.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>