drm/mediatek: clear IRQ status before enable OVL interrupt

To make sure that the first vblank IRQ after enabling
vblank isn't too short or immediate, we have to clear
the IRQ status before enable OVL interrupt.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Acked-by: CK Hu <ck.hu@mediatek.com>