drm/rockchip: lvds: improve error handling in helper functions

Return errors instead of returning void from internal helpers. When
these helpers are called, check the returned value and print an error
message in this case and not blindly continue.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224143900.23567-8-miquel.raynal@bootlin.com