drm/tinydrm: add backlight dependency for ili9341

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