dmaengine: consolidate memcpy apis

Copying from page to page (dma_async_memcpy_pg_to_pg) is the superset,
make the other two apis use that one in preparation for providing a
common dma unmap implementation.  The common implementation just wants
to assume all buffers are mapped with dma_map_page().

Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Tomasz Figa <t.figa@samsung.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>