drm/rockchip: vop: Init vskiplines in scl_vop_cal_scale()

Currently we are calling scl_vop_cal_scale() to get vskiplines for yrgb
and cbcr. So the cbcr's vskiplines might be an unexpected value if the
second scl_vop_cal_scale() didn't update it.

Init vskiplines in scl_vop_cal_scale() to avoid that.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180223062250.10470-1-jeffy.chen@rock-chips.com