fbdev: sh_mobile_lcdc: Split channel initialization from probe function

Move channel initialization to sh_mobile_lcdc_channel_init() and call
the function from sh_mobile_lcdc_probe(). This makes the code more
readable and prepares it for fix/var initialization rework.

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