gpu: ipu-v3: Move IDMAC channel names to imx-ipu-v3.h

Move the IDMAC channel names to imx-ipu-v3.h, to make the names
available outside IPU. Add a couple new channels in the process
(async display BG/FG, channels 24 and 29).

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>