fbdev: sh_mobile_lcdcfb: allocate memory, sufficient for the biggest mode

Now, that we add support for multiple modes to the sh_mobile_lcdcfb driver, it
has to allocate memory, sufficient for the biggest of them.

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