imx-drm: ipu-v3: don't use clk_round_rate() before clk_set_rate()

This is nonsense; clk_round_rate() is just clk_set_rate() without the
side effect of changing the hardware.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>