drm/mediatek/mtk_disp_color: Strip incorrect doc and demote header

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

 drivers/gpu/drm/mediatek/mtk_disp_color.c:45: warning: Function parameter or member 'clk' not described in 'mtk_disp_color'
 drivers/gpu/drm/mediatek/mtk_disp_color.c:45: warning: Function parameter or member 'regs' not described in 'mtk_disp_color'
 drivers/gpu/drm/mediatek/mtk_disp_color.c:45: warning: Function parameter or member 'cmdq_reg' not described in 'mtk_disp_color'

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-mediatek@lists.infradead.org
Cc: linux-arm-kernel@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/20210602143300.2330146-2-lee.jones@linaro.org