drm/mediatek: Fix device passed to cmdq

drm device is now probed from mmsys. We need to use mmsys device to get gce
nodes. Fix following errors:

[    0.740068] mediatek-drm mediatek-drm.1.auto: error -2 can't parse gce-client-reg property (0)
[    0.748721] mediatek-drm mediatek-drm.1.auto: error -2 can't parse gce-client-reg property (0)
...
[    2.659645] mediatek-drm mediatek-drm.1.auto: failed to request channel
[    2.666270] mediatek-drm mediatek-drm.1.auto: failed to request channel

Fixes: 667c7692 ("soc / drm: mediatek: Fix mediatek-drm device probing")
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>