drm: omapdrm: dss: replace ternary operator with max()

Fix the following coccicheck warning:

drivers/gpu/drm/omapdrm/dss/dispc.c:2454:21-22: WARNING opportunity for max()

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220517050208.121488-1-guozhengkui@vivo.com