video: fbdev: sh_mobile_lcdcfb: Remove redundant null check before clk_prepare_enable/clk_disable_unprepare

Because clk_prepare_enable() and clk_disable_unprepare() already checked
NULL clock parameter, so the additional checks are unnecessary, just
remove them.
Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201014085722.26069-1-vulab@iscas.ac.cn