drm/mediatek: Fix config_updating flag never false when no mbox channel

When CONFIG_MTK_CMDQ is enabled, if the display is controlled by the CPU
while other hardware is controlled by the GCE, the display will encounter
a mbox request channel failure.
However, it will still enter the CONFIG_MTK_CMDQ statement, causing the
config_updating flag to never be set to false. As a result, no page flip
event is sent back to user space, and the screen does not update.

Fixes: da03801 ("drm/mediatek: Move mtk_crtc_finish_page_flip() to ddp_cmdq_cb()")
Signed-off-by: Jason-JH Lin <jason-jh.lin@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250224051301.3538484-1-jason-jh.lin@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

Author: Jason-JH Lin

drivers/gpu/drm/mediatek/mtk_crtc.c

@@ -620,13 +620,16 @@ static void mtk_crtc_update_config(struct mtk_crtc *mtk_crtc, bool needs_vblank)
 
 		mbox_send_message(mtk_crtc->cmdq_client.chan, cmdq_handle);
 		mbox_client_txdone(mtk_crtc->cmdq_client.chan, 0);
+		goto update_config_out;
 	}
-#else
+#endif
 	spin_lock_irqsave(&mtk_crtc->config_lock, flags);
 	mtk_crtc->config_updating = false;
 	spin_unlock_irqrestore(&mtk_crtc->config_lock, flags);
-#endif
 
+#if IS_REACHABLE(CONFIG_MTK_CMDQ)
+update_config_out:
+#endif
 	mutex_unlock(&mtk_crtc->hw_lock);
 }