drm/rockchip: lvds: Change platform data to hold helper_funcs pointer

Prepare the introduction of PX30 support by using
drm_encoder_helper_funcs as platform data instead of multiple register
names which are specific to rk3288 and not generic to all Rockchip
IPs. This way adding support for a new flavor of a similar IP will be
a matter of adding the relevant helper funcs.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224143900.23567-6-miquel.raynal@bootlin.com