video: mxsfb: convert to clk_prepare/clk_unprepare

The patch converts mxsfb driver to clk_prepare/clk_unprepare by
using helper functions clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>