drm/rockchip: Make analogix_dp_atomic_check static

Fix sparse warning:

drivers/gpu/drm/bridge/analogix/analogix_dp_core.c:1151:5: warning:
 symbol 'analogix_dp_atomic_check' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730150057.57388-1-yuehaibing@huawei.com