drm: rcar-du: lvds: Remove LVDS double-enable checks

The DRM core and DU driver guarantee that the LVDS bridge will not be
double-enabled or double-disabled. Remove the corresponding unnecessary
checks.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>