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: d1a2e7004b5e ("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

drivers/gpu/drm/tinydrm/Kconfig

@@ -23,6 +23,7 @@ config TINYDRM_ILI9225
 config TINYDRM_MI0283QT
 	tristate "DRM support for MI0283QT"
 	depends on DRM_TINYDRM && SPI
+	depends on BACKLIGHT_CLASS_DEVICE
 	select TINYDRM_MIPI_DBI
 	help
 	  DRM driver for the Multi-Inno MI0283QT display panel
@@ -54,6 +55,7 @@ config TINYDRM_ST7586
 config TINYDRM_ST7735R
 	tristate "DRM support for Sitronix ST7735R display panels"
 	depends on DRM_TINYDRM && SPI
+	depends on BACKLIGHT_CLASS_DEVICE
 	select TINYDRM_MIPI_DBI
 	help
 	  DRM driver Sitronix ST7735R with one of the following LCDs: