drm/rockchip: lvds: Harmonize function names

Prepare the introduction of PX30 support by clarifying the function
prefixes.

We continue to prefix with 'rockchip_lvds_' generic functions that are
not specific to a single hardware. Functions implying hardware
modifications are now prefixed with 'rk3288_lvds_'.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224143900.23567-5-miquel.raynal@bootlin.com