drm: xlnx: Use dma_request_chan for DMA channel request

There is no need to use the of_dma_request_slave_channel() directly as
dma_request_chan() is going to try to get the channel via OF as well.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201023094602.5630-1-peter.ujfalusi@ti.com