OMAPDSS: Add missing dependency on backlight for DSI-CM panel drier

The DSI-CM driver uses the backlight class so needs to build depend on it.

Signed-off-by: Mark Brown <broonie@linaro.org>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>