fbdev: sh_mobile_lcdc: increase maximum framebuffer size to support 1080p

LCDC hardware can support 1920x1080 formats, adjust the driver to cover them.
Besides, instead of guessing some "reasonable" validity checks, only verify
values in .fb_check_var(), that we are sure, we cannot support.

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