drm/rockchip: dsi: make hstt_table static

This symbol is not used outside of dw-mipi-dsi-rockchip.c, so marks
it static.

Fix the following sparse warning:

drivers/gpu/drm/rockchip/dw-mipi-dsi-rockchip.c:646:13: warning: symbol
'hstt_table' was not declared. Should it be static?

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1628218664-14230-1-git-send-email-jiapeng.chong@linux.alibaba.com