dmaengine: dw: introduce dwc_dostart_first_queued() helper

We have a duplicate code which starts first descriptor in the queue. Let's make
this as a separate helper that can be used in future as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>