dma-mapping: remove dma_{alloc,free,mmap}_writecombine

We can already use DMA_ATTR_WRITE_COMBINE or the _wc prefixed version,
so remove the third way of doing things.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>