video: ARM CLCD: use panel device node for getting backlight and mode

Having obtained panel device node in clcdfb_of_init_display() it allows to
generalize and simplify two more helper functions clcdfb_of_get_backlight()
and clcdfb_of_get_mode().

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>