drm/mediatek/mtk_dpi: Remove unused struct definition 'mtk_dpi_encoder_funcs'

drm/mediatek/mtk_dpi: Remove unused struct definition 'mtk_dpi_encoder_funcs'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/mediatek/mtk_dpi.c:530:39: warning: ‘mtk_dpi_encoder_funcs’ defined but not used [-Wunused-const-variable=]

Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Jie Qiu <jie.qiu@mediatek.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-8-lee.jones@linaro.org