drm/rockchip: vop: call vop_cfg_done() under reg_lock

The function vop_cfg_done() is a simple VOP_REG_SET(). As such it should
be done under a reg_lock. A quick look through the driver shows that all
other instances (apart from driver init) have the lock. Do the same here

Cc: Sandy Huang <hjc@rock-chips.com>
Cc: Heiko Stübner <heiko@sntech.de>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Sandy Huang <hjc@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200505151613.2932456-1-emil.l.velikov@gmail.com