drm/rockchip: remove unused function

Fix the following clang warning:

drivers/gpu/drm/rockchip/dw-mipi-dsi-rockchip.c:320:20: warning: unused
function 'dsi_set' [-Wunused-function].

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/1618476421-114429-1-git-send-email-jiapeng.chong@linux.alibaba.com