drm: panel/simple: add backlight dependency

The simple panel code uses the backlight interface to
find a device, which fails when backlight is disabled:

drivers/built-in.o: In function `panel_simple_platform_probe':
:(.text+0xd3c48): undefined reference to `of_find_backlight_by_node'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@gmail.com>

drivers/gpu/drm/panel/Kconfig

@@ -10,6 +10,7 @@ menu "Display Panels"
 config DRM_PANEL_SIMPLE
 	tristate "support for simple panels"
 	depends on OF
+	depends on BACKLIGHT_CLASS_DEVICE
 	help
 	  DRM panel driver for dumb panels that need at most a regulator and
 	  a GPIO to be powered up. Optionally a backlight can be attached so