drm/panel: S6E3HA2 needs backlight code

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: ed29f9426d9b ("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