v4l: xilinx: scd: Initialize DMA channels in xilinx-scenechange-dma.c

v4l: xilinx: scd: Initialize DMA channels in xilinx-scenechange-dma.c

Most of the DMA channel initialization is performed in
xilinx-scenechange-dma.c, but a small part is in
xilinx-scenechange-channel.c. Move all the initialization code to
xilinx-scenechange-dma.c, rename the xscd_dma_chan_probe() function to
xscd_dma_chan_init(), and simplify the code as the function can't fail.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>