drm/panel: Convert sysfs sprintf/snprintf family to sysfs_emit

Fix the following coccicheck warning:
drivers/gpu/drm/panel//panel-tpo-td043mtea1.c:217:8-16: WARNING:
use scnprintf or sprintf
drivers/gpu/drm/panel//panel-tpo-td043mtea1.c:189:8-16: WARNING:
use scnprintf or sprintf

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1617069288-8317-1-git-send-email-tiantao6@hisilicon.com