drm/mediatek: add CONFIG_OF dependency

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>