drm/rockchip: Add support for Rockchip Soc RGB output interface

Some Rockchip CRTCs, like rv1108 and px30, can directly output parallel
and serial RGB data to panel or conversion chip.

So add a feature-bit for vops to mark the ability for these direct
outputs and add an internal encoder in that case, that can attach to
bridge chipsor panels.

Changes in v7:
    1. forget to delete rockchip_rgb_driver and delete it.
Changes in v6:
    1. Update according to Heiko Stuebner' implemention, rgb output is
       part of vop's feature, should not register as a independent
       driver.
Changes in v5:
    1. add SPDX-License-Identifier tag
Changes in v4:
    1. add support px30;
Changes in v3:
    1. update for rgb-mode move to panel node.
Changes in v2:
    1. add error log when probe failed;
    2. update name_to_output_mode() according to sean's suggest;
    3. Fix uninitialized use of ret.

Signed-off-by: Sandy Huang <hjc@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/20180830211207.10480-3-heiko@sntech.de