drm/rockchip: Embed drm_encoder into rockchip_decoder

The VOP2 driver needs rockchip specific information for a drm_encoder.

This patch creates a struct rockchip_encoder with a struct drm_encoder
embedded in it. This is used throughout the rockchip driver instead of
struct drm_encoder directly.

The information the VOP2 drivers needs is the of_graph endpoint node
of the encoder. To ease bisectability this is added here.

While at it convert the different encoder-to-driverdata macros to
static inline functions in order to gain type safety and readability.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220422072841.2206452-3-s.hauer@pengutronix.de