media: platform: mdp: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/media/platform/mediatek/mdp/mtk_mdp_core.c:31:34: error: ‘mtk_mdp_comp_dt_ids’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/platform/mediatek/mdp/mtk_mdp_core.c

@@ -28,7 +28,7 @@ EXPORT_SYMBOL(mtk_mdp_dbg_level);
 
 module_param(mtk_mdp_dbg_level, int, 0644);
 
-static const struct of_device_id mtk_mdp_comp_dt_ids[] = {
+static const struct of_device_id mtk_mdp_comp_dt_ids[] __maybe_unused = {
 	{
 		.compatible = "mediatek,mt8173-mdp-rdma",
 		.data = (void *)MTK_MDP_RDMA