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: NBibby Hsieh <bibby.hsieh@mediatek.com>
Acked-by: NCK Hu <ck.hu@mediatek.com>

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -217,6 +217,7 @@ static int mtk_drm_kms_init(struct drm_device *drm)
 	if (ret < 0)
 		goto err_component_unbind;
 
+	drm->vblank_disable_allowed = true;
 	drm_kms_helper_poll_init(drm);
 	drm_mode_config_reset(drm);