drm/rockchip: cdn-dp: correct rate in the struct drm_dp_link assignment

The value stored in the rate field of struct drm_dp_link should
be the actual link-rate and not the bw_code.

Right now the driver stores the code and converts it to the rate.
So fixup the driver to store the rate itself.

Signed-off-by: Sandor Yu <Sandor.yu@nxp.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605080424.28731-1-sandor.yu@nxp.com