fbdev: sh_mobile_lcdc: Adjust requested parameters in .fb_check_var

Instead of failing when the requested fb_var_screeninfo parameters are
not supported, adjust the parameters according to the hardware
capabilities.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>