dmatest: restore ability to start test at module load and init

1/ move 'run' control to a module parameter so we can do:
   modprobe dmatest run=1.  With this moved the rest of the debugfs
   boilerplate can go.

2/ Fix parameter initialization.  Previously the test was being started
   without taking the parameters into account in the built-in case.

Also killed off the '__' version of some routines.  The new rule is just
hold the lock when calling a *threaded_test() routine.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>