mailbox: mediatek: cmdq: move the CMDQ_IRQ_MASK into cmdq driver data

The interrupt mask and thread number has positive correlation,
so we move the CMDQ_IRQ_MASK into cmdq driver data and calculate
it by thread number.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>