drm/mediatek: set vblank_disable_allowed to true

MTK DRM driver didn't set the vblank_disable_allowed to
true, it cause that the irq_handler is called every
16.6 ms (every vblank) when the display didn't be updated.

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