video: mxsfb: add clk_enable()

BROM doesn't enable lcdif by default so add clk_enable() after
clk_set_rate().

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>