fbdev: sh_mobile_lcdc: Don't store copy of platform data

Instead of copying the whole platform data structure to struct
sh_mobile_lcdc_chan, store a const pointer to the channel platform data.

MERAM configuration information needs to be changed at runtime, so copy
it to struct sh_mobile_lcdc_chan.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>