mode_valid function is missing for lvds.

Add it making it pointed by encoder helper functions.
Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1520940019-68977-1-git-send-email-giulio.benetti@micronovasrl.com