fbdev: sh_mobile_lcdcfb: use fb_videomode_to_var() instead of open-coding

Use the standard fb_videomode_to_var() instead of filling fb_var_screeninfo
fields manually in, also remove a redundant memset(0).

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>