drm/panel: ili9322: Make local symbols static

Fixes the following sparse warnings:

drivers/gpu/drm/panel/panel-ilitek-ili9322.c:182:12: warning:
 symbol 'ili9322_inputs' was not declared. Should it be static?
drivers/gpu/drm/panel/panel-ilitek-ili9322.c:343:28: warning:
 symbol 'ili9322_regmap_config' was not declared. Should it be static?

Also change ili9322_inputs to 'const char * const' to avoid
chackpatch warning.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1514948938-19996-1-git-send-email-weiyongjun1@huawei.com