drm/mediatek: Use drm_atomic destroy_state helpers

Use the core destroy_state helpers to destroy core state to ensure we don't
leak if/when more fields get added later.

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/1470279597-60453-5-git-send-email-bibby.hsieh@mediatek.com