drm/mediatek: mtk_dpi: Fix unused variable 'mtk_dpi_encoder_funcs'

Commit f89c696e ("drm/mediatek: mtk_dpi: Convert to bridge driver")
introduced the following build warning with W=1

 drivers/gpu/drm/mediatek/mtk_dpi.c:530:39: warning: unused variable 'mtk_dpi_encoder_funcs' [-Wunused-const-variable]
 static const struct drm_encoder_funcs mtk_dpi_encoder_funcs = {

This struct is and the 'mtk_dpi_encoder_destroy()' are not needed
anymore, so remove them.

Fixes: f89c696e ("drm/mediatek: mtk_dpi: Convert to bridge driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>