tinydrm: add backlight dependency

Calling devm_of_find_backlight directly means we get a link failure
without CONFIG_BACKLIGHT_CLASS_DEVICE:

drivers/gpu/drm/tinydrm/mi0283qt.o: In function `mi0283qt_probe':
mi0283qt.c:(.text+0x684): undefined reference to `devm_of_find_backlight'

This adds an explicit Kconfig dependency for it. While I did not
observe that failure for st7735r, I assume the same change is needed
there for the same reason.

Fixes: d1a2e700 ("drm/tinydrm: Replace tinydrm_of_find_backlight with of_find_backlight")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180228134111.2042877-1-arnd@arndb.de