fbdev: sh_mobile_lcdcfb: separate display variable data from framebuffer data

This is a preparation for a patch, that shall allow displaying of a smaller
framebuffer on a bigger display and of a part of a bigger framebuffer on a
smaller display.

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