drm/bridge: tc358768: select CONFIG_VIDEOMODE_HELPERS

A dependency on this feature was recently introduced:

x86_64-linux-ld: vmlinux.o: in function `tc358768_bridge_pre_enable':
tc358768.c:(.text+0xbe3dae): undefined reference to `drm_display_mode_to_videomode'

Make sure this is always enabled.

Fixes: e5fb2167 ("drm/bridge: tc358768: Use struct videomode")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20231204072814.968816-1-arnd@kernel.orgSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231204072814.968816-1-arnd@kernel.org

drivers/gpu/drm/bridge/Kconfig

@@ -313,6 +313,7 @@ config DRM_TOSHIBA_TC358768
 	select REGMAP_I2C
 	select DRM_PANEL
 	select DRM_MIPI_DSI
+	select VIDEOMODE_HELPERS
 	help
 	  Toshiba TC358768AXBG/TC358778XBG DSI bridge chip driver.