drm/tinydrm: add backlight dependency for ili9341

This tinydrm driver fails to link without the backlight support:

drivers/gpu/drm/tinydrm/ili9341.o: In function `ili9341_probe':
ili9341.c:(.text+0x578): undefined reference to `devm_of_find_backlight'

Fixes: 3fa0e8f6f960 ("drm/tinydrm: new driver for ILI9341 display panels")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709152106.990066-1-arnd@arndb.de

drivers/gpu/drm/tinydrm/Kconfig

@@ -23,6 +23,7 @@ config TINYDRM_ILI9225
 config TINYDRM_ILI9341
 	tristate "DRM support for ILI9341 display panels"
 	depends on DRM_TINYDRM && SPI
+	depends on BACKLIGHT_CLASS_DEVICE
 	select TINYDRM_MIPI_DBI
 	help
 	  DRM driver for the following Ilitek ILI9341 panels: