drm/mediatek: Use cmdq_pkt_create() and cmdq_pkt_destroy()

Use cmdq_pkt_create() and cmdq_pkt_destroy() common function
instead of implementing drm version.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240810090918.7457-3-chunkuang.hu@kernel.org/