drm/omap: dmm_tiler: Use dmaengine_prep_dma_memcpy() for i878 workaround

Instead of dma_dev->device_prep_dma_memcpy() use the existing macro to
prepare the memcpy.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190731094233.13890-2-peter.ujfalusi@ti.com