dmatest: create dmatest_info to keep test parameters

The proposed change will remove usage of the module parameters as global
variables. In future it helps to run different test cases sequentially.

The patch introduces the run_threaded_test() and stop_threaded_test() functions
that could be used later outside of dmatest_init, dmatest_exit scope.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>