drm/omap: Depend on CONFIG_OF

The driver accesses the drm_bridge.of_node field, which is present only
if CONFIG_OF is enabled. As all platforms using omapdrm are OF-based, we
can simply depend on CONFIG_OF.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/omapdrm/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config DRM_OMAP
 	tristate "OMAP DRM"
-	depends on DRM
+	depends on DRM && OF
 	depends on ARCH_OMAP2PLUS || ARCH_MULTIPLATFORM
 	select DRM_KMS_HELPER
 	select VIDEOMODE_HELPERS