drm/mediatek/mtk_disp_ovl: Fix formatting and provide missing member description

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

 drivers/gpu/drm/mediatek/mtk_disp_ovl.c:75: warning: Function parameter or member 'ddp_comp' not described in 'mtk_disp_ovl'
 drivers/gpu/drm/mediatek/mtk_disp_ovl.c:75: warning: Function parameter or member 'crtc' not described in 'mtk_disp_ovl'
 drivers/gpu/drm/mediatek/mtk_disp_ovl.c:75: warning: Function parameter or member 'data' not described in 'mtk_disp_ovl'

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: 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/20201116174112.1833368-10-lee.jones@linaro.org