gpu: ipu-di: Switch to DIV_ROUND_CLOSEST for DI clock divider calc

We can use the DIV_ROUND_CLOSEST() macro when calculating the DI
clock divider, rounded to nearest int.

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