drm/panel: S6E3HA2 needs backlight code

The new S6E3HA2 driver fails to link when backlight is disabled:

ERROR: "backlight_device_register" [drivers/gpu/drm/panel/panel-samsung-s6e3ha2.ko] undefined!
ERROR: "backlight_device_unregister" [drivers/gpu/drm/panel/panel-samsung-s6e3ha2.ko] undefined!

This adds a Kconfig dependency like we have it for some other panel drivers.

Fixes: ed29f942 ("drm/panel: Add support for S6E3HA2 panel driver on TM2 board")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170419175939.189098-2-arnd@arndb.de

drivers/gpu/drm/panel/Kconfig

@@ -77,6 +77,7 @@ config DRM_PANEL_SAMSUNG_S6E3HA2
 	tristate "Samsung S6E3HA2 DSI video mode panel"
 	depends on OF
 	depends on DRM_MIPI_DSI
+	depends on BACKLIGHT_CLASS_DEVICE
 	select VIDEOMODE_HELPERS
 
 config DRM_PANEL_SAMSUNG_S6E8AA0