drm/mediatek: Add pm_runtime support for GCE power control

drm/mediatek: Add pm_runtime support for GCE power control

Call pm_runtime_resume_and_get() before accessing GCE hardware in
mbox_send_message(), and invoke pm_runtime_put_autosuspend() in the
cmdq callback to release the PM reference and start autosuspend for
GCE. This ensures correct power management for the GCE device.

Fixes: 8afe816b0c99 ("mailbox: mtk-cmdq-mailbox: Implement Runtime PM with autosuspend")
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/20250829091727.3745415-3-jason-jh.lin@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>