MAINTAINERS: change maintainer for Rockchip drm drivers

For personal reasons, Mark Yao will leave rockchip,
can not continue maintain drm/rockchip, Sandy Huang
and Heiko Stübner will take over drm/rockchip.

Cc: Sandy Huang <hjc@rock-chips.com>
Cc: Heiko Stübner <heiko@sntech.de>

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
[seanpaul added Heiko]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171129142459.32351-1-seanpaul@chromium.org