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

drivers/gpu/drm/rockchip/rockchip_rgb.h

@@ -0,0 +1,20 @@
+//SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) Fuzhou Rockchip Electronics Co.Ltd
+ * Author:
+ *      Sandy Huang <hjc@rock-chips.com>
+ *
+ * This software is licensed under the terms of the GNU General Public
+ * License version 2, as published by the Free Software Foundation, and
+ * may be copied, distributed, and modified under those terms.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+struct rockchip_rgb *rockchip_rgb_init(struct device *dev,
+				       struct drm_crtc *crtc,
+				       struct drm_device *drm_dev);
+void rockchip_rgb_fini(struct rockchip_rgb *rgb);