drm/rockchip: vop: fix NV12 video display error

fixup the scale calculation formula on the case
src_height == (dst_height/2).

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Sandy huang <sandy.huang@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1501494586-6984-1-git-send-email-mark.yao@rock-chips.com