video/omap: fix build dependencies

Four of the LCD panel drivers depend on the backlight class,
so add the dependency in Kconfig.
Selecting the BACKLIGHT_CLASS_DEVICE symbol does not generally
work since it has other dependencies.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[tomi.valkeinen@ti.com: changed also N8x0 panel]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>