drm/mediatek: fix a typo of OD_CFG to OD_RELAYMODE

If we want to set the hardware OD to relay mode,
we have to set OD_CFG register rather than
OD_RELAYMODE; otherwise, the system will access
the wrong address.

Fixes: 72164364

 ("drm/mediatek: set mt8173 dithering function")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Acked-by: CK Hu <ck.hu@mediatek.com>