drm/rockchip: support prime import sg table

The prime fd to handle ioctl was not used with rockchip before. Support
was added in order to pass graphics_Gbm and to support potential uses
within Chrome OS (e.g. zero-copy video decode, camera).

Signed-off-by: Haixia Shi <hshi@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180130202913.28724-3-thierry.escande@collabora.com