drm/panel: st7703: Make jh057n00900_panel_desc static

This eliminates the following sparse warning:

drivers/gpu/drm/panel/panel-sitronix-st7703.c:156:26: warning: symbol
'jh057n00900_panel_desc' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
[Use static const]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200912033809.142773-1-yanaijie@huawei.com