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: da03801a ("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>