This is a follow up to the commit cf65a0f6

  ("dma-mapping: move all DMA mapping code to kernel/dma")

which moved source code of DMA API to kernel/dma folder. Since there is
no file left in the lib that require DMA API debugging options move the
latter to kernel/dma as well.

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>