drm/panel: dsi-cm: replace snprintf in show functions with sysfs_emit

show() must not use snprintf() when formatting the value to be
returned to user space.

Fix the following coccicheck warning:
drivers/gpu/drm/panel/panel-dsi-cm.c:251: WARNING: use scnprintf or sprintf.
drivers/gpu/drm/panel/panel-dsi-cm.c:271: WARNING: use scnprintf or sprintf.

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Signed-off-by: Qing Wang <wangqing@vivo.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1634280490-4429-1-git-send-email-wangqing@vivo.com