We could use DEFINE_IDR for statically allocated idr
that allow us to save a few lines of code.

And also remove unneeded mutex_init() for dma_list_mutex, as
dma_list_mutex is initialized automatically by DEFINE_MUTEX().
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>