drm/mediatek: Remove mtk_drm_crtc_check_flush

This function no longer exists.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1470279597-60453-2-git-send-email-bibby.hsieh@mediatek.com