drm/mediatek: add CONFIG_OF dependency

The mediatek DRM driver can be configured for compile testing with
CONFIG_OF disabled, but then fails to link:

drivers/gpu/built-in.o: In function `mtk_drm_bind':
analogix_dp_reg.c:(.text+0x52888): undefined reference to `of_find_device_by_node'
analogix_dp_reg.c:(.text+0x52930): undefined reference to `of_find_device_by_node'

This adds an explicit Kconfig dependency.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patchwork.kernel.org/patch/9120871/Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/drm/mediatek/Kconfig

@@ -3,6 +3,7 @@ config DRM_MEDIATEK
 	depends on DRM
 	depends on ARCH_MEDIATEK || (ARM && COMPILE_TEST)
 	depends on COMMON_CLK
+	depends on OF
 	select DRM_GEM_CMA_HELPER
 	select DRM_KMS_HELPER
 	select DRM_MIPI_DSI