gpu: ipu-v3: Add ipu_idmac_enable_watermark()

Adds the function ipu_idmac_enable_watermark(), which enables or disables
watermarking in the IDMAC channel. Enabling watermarking can increase a
channel's AXI bus arbitration priority.

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