mailbox: mtk-cmdq-mailbox: Use platform data directly instead of copying

Copying platform data to struct cmdq serves to no purpose, as that
data is never modified during runtime: it's worth at this point
storing a pointer to gce_plat in gce and.

Remove all duplicated `struct gce_plat` members from `struct gce`
and reuse the platform data across the driver to save some memory.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>