gpu: ipu-v3: smfc: Add ipu_smfc_set_watermark()

Adds ipu_smfc_set_watermark() which programs a channel's SMFC FIFO
levels at which the watermark signal is set and cleared.

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