media: ti: j721e-csi2rx: allocate DMA channel based on context index

media: ti: j721e-csi2rx: allocate DMA channel based on context index

With multiple contexts, there needs to be a different DMA channel for
each context. Earlier, the DMA channel name was hard coded to "rx0" for
the sake of simplicity. Generate the DMA channel name based on its index
and get the channel corresponding to the context.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Jai Luthra <j-luthra@ti.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Yemike Abhilash Chandra <y-abhilashchandra@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Rishikesh Donadkar <r-donadkar@ti.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>